The Finns had cut all the roads to the motti, but were unable to form a tight blockade in the thick forests, which allowed most of the 115^th Division and 123^rd Division to escape toward Koivisto. However, the bulk of the 43^rd Division was destroyed at the Battle of Porlammi on 1 September. The Finns force marched to the village and port of Koivisto on 2 September, but did not pursue the remnants of the Soviet divisions, which had fled to the islands of the surrounding archipelago: in November, these units were evacuated by the Soviets. While the fighting near Viipuri still raged, the Finnish advance toward Leningrad continued. The IV Armeijakunta was to advance along the western shore, the II Armeijakunta in the centre and Eversti Einar Mäkinen’s newly arrived I Armeijajunta along the eastern side of the isthmus. Mannerheim ordered the Finnish advance to halt short of the Soviet fortifications, and by 31 August these forces had reached the old border, and early in September arrived in front of the the Soviet fortifications and ceased their advance.

On 20 August, General Dr Waldemar Erfurth, a staff officer of the Oberkommando des Heeres and a member of the German liaison staff in Finland, notified Mannerheim that Generalfeldmarschall Wilhelm Keitel was about to send a letter detailing the point at which the Finns were to be asked to attack Leningrad. Mannerheim responded with the practical difficulties of the proposal and presented the Finnish political and military leadership’s opposition to any such attack. The government had decided beforehand that Finland would not attack Leningrad, and it was only after pressure from the military leadership that they accepted a small advance across the old border with the object of taking better defensive positions. The Social Democrat part in particular opposed any crossing the border. When Keitel’s letter arrived, came, President Risto Heikki Ryti and Mannerheim together prepared a negative answer. On 31 August, Erfurth contacted Mannerheim again and proposed that the Finns should cancel the attack into East Karelia, and instead attack Leningrad, but once more Ryti and Mannerheim refused. On 31 August Mannerheim gave the order that the attack be stopped along the line linking Ohta and the mouth of the Rajajoki river. The exact line between Ohta and Lake Ladoga would be determined at a later date after the Finns had reached the old border in that area. That would shorten the front without the need to attack Soviet fortifications to the north of Leningrad in the 2^nd Karelian Fortified Region. In this last phase, the Soviets had six infantry divisions and a number of separate regiments and battalions to defend Leningrad against an attack from the north-west, but all of the Soviet elements were at only half strength as a result of the hard fighting with the Finns.

The 12^th Divisioona had reached its objective by 1 September, but elsewhere the attack started on 2 September. The 18^th Divisioona captured Mainila on this same day and Valkeasaari (now Beloostrov) on the following day. By 7 September both the 18^th Divisioona and the 2^nd Divisioona had reached their targets between the Rajajoki river and Ohta. Mäkinen, the commander of the I Armeijakunta, ordered his men to advance to the line linking Ohta and the Lempaalanjärvi, the old border, at Lake Ladoga with the instruction that, in the event that his troops met strong opposition, the offensive could be stopped there. On 4 September the attack began, and on 6 September the 10^th Divisioona managed to encircle and destroy the Soviet 941^st Regiment at Kirjasalo. Finally, on 9 September the Finns reached their objective line at every point and switched to a defensive posture.

The Soviet military leadership quickly learned of the diminished Finnish pressure, and by 5 September two divisions were already transferred from the Karelian isthmus to the area to the south of Leningrad against the German onslaught from the south. Although the Finnish troops on the Karelian isthmus did not play any active role on the forthcoming siege of Leningrad, their mere presence contributed to the siege by hampering the supply of the city around and across Lake Ladoga.

At the same time as the fighting on the Karelian isthmus, other Finnish forces were committed to an invasion of Ladoga Karelia, the south-eastern part of Finland between the Karelian isthmus and East Karelia round the north-eastern corner of Lake Ladoga. Initially, the Finnish army was deployed in a defensive posture, but on 29 June Mannerheim created the Karjalan Armeija under the command of Heinrichs, and ordered this new formation to prepare to take the offensive into Ladoga Karelia. The new army comprised Kenraalimajuri Paavo Talvela’s VI Armeijakunta with Eversti Ruben Lagus’s (from 25 July Eversti Ilmari Karhu’s 5^th Divisioona and Eversti Kaarlo Heiskanen’s 11^th Divisioona, Kenraalimajuri Woldemar Hägglund’s VII Armeijajunta with Eversti Antero Svensson’s 7^th Divisioona and Eversti Hannu Hannuksela’s 19^th Divisioona, and Kenraalimajuri Woldemar Oinonen’s Ryhmä 'Oinonen' (also known as Ryhmä 'O', Ratsuväkiprikaati (cavalry brigade), 1^st Jääkäriprikaati and 2^nd Jääkäriprikaati). Eversti Paavo Paalu’s 1^st Divisioona was the corps' reserve. The Finnish plan was based on an advance to divide the defending Soviet forces by reaching the north-west shore of Lake Ladoga and then advance along the shore of the lake.

Opposing the VII Armeijakunta was Gorelenko’s 7^th Army with the 168^th Division near Sortavala and the 71^st Division to the north of the Jänisjärvi lake. The Soviets had prepared field fortifications along the border in the area of Sortavala and at the important road junctions at Värtsilä and Korpiselkä.

It was on 9 July that the order to begin the Finnish offensive was given, and the main task of the VI Armeijakunta was to break through the Soviet defences between Värtsilä and Korpiselkä. The Finnish offensive quickly overwhelmed the Soviet defenders. The 1^st Jääkäriprikaati was switched from the Ryhmä 'Oinonen' to spearhead the assault, and this unit opened a gap to the Soviet defences through which Finnish light infantry, some mounted on bicycles, pushed forward.

The right flank of the Finnish offensive, based on the 11^th Divisioona, encountered strong Soviet resistance on the eastern shore of the Jänisjärvi lake, and the clearance of this resistance lasted to 16 July. The 11^th Divisioona then continued its planned advance and rounded the southern end of the Jänisjärvi lake to establish westward-facing positions along the Jänisjoki river. At the same time, Hägglund’s VII Corps had been attacking to the south on the western side of the Jänisjärvi lake, but the determined nature of the Soviet defence made the offensive little more than a crawl. It was thus only on 15 July, therefore, that the Finnish forces reached the main Soviet defences. It took until 17 July for the VII Armeijakunta finally to reach the Jänisjoki river, and clearing the surrounded Soviet forces lasted until 21 July. Since the Finnish advance had extended the front, some of the Finnish forces were starting to redeploy on 16 July, with the 1^st Divisioona being ordered to cover the eastern flank of the advance while Eversti Aarne Snellman’s 17^th Divisioona, which had handed responsibility for the blockade of the Soviet base at Hanko in the north-west coast of the Gulf of Finland to local troops, was also brought into the area. Generalleutnant Erwin Engelbrecht’s two-regiment German 163^rd Division was ordered to capture the town and railroad junction of Suvilahti. These formation adjustments in effect increased the Finnish strength in the area by three divisions.

The advance on the left flank of Talvela’s VI Armeijakunta by the two-brigade Ryhmä 'Oinonen' stalled almost as soon as it had started. Its advance tied down some Soviet troops, but Talvela assessed that the Ryhmä 'Oinonen''s mission had been a resounding failure. However, he also criticised his superior’s orders to use these lighter troops against Soviet positions which were already known to be strong.

The main Finnish advance continued to the south in the direction of Loimola, through which ran the railway linking Sortavala and Petrozavodsk. Loimola had fallen to the Finnish forces by 15 July. Talvela pressed his forces farther and the 1^st Jääkäriprikaati completed its 68-mile (110-km) contested advance when it reached the shore of Lake Ladoga at Koirinoja on the following day. This severed the connections between the Soviet forces in the area. While Talvela’s units continued their advances farther to the east along the shore of the Lake Ladoga as well as farther inland, the Soviets had reorganised some of their forces and were rushing reinforcements to Lake Ladoga’s eastern shore. The 452^nd Motorised Regiment established defensive positions around the town of Salmi, but the Finnish advance encircled the defenders and the Finns had taken Salmi by 21 July. After the VI armeijakunta reached the 1939 border on 23 July, Mannerheim on the next day ordered a halt to any advances farther to the east and set the Finnish forces to the preparation of defensive positions along the Tuulema river. Crossing of the 1939 border did not sit well with all of the Finns, and more than 2,000 men initially refused to set foot across the old border.

The 7^th Divisioona of the VII Armeijakunta launched its attack toward Sortavala from the east and managed to capture the village of Ruskeala on 25 July, so making it possible for the Finns to present a unified front against Soviet forces holding Sortavala. For their part, the Soviets had reinforced the the defence by the 168^th Division with the 198^th Motorised Division and prepared to launch a counterattack toward Jänisjoki river. However, the Finns captured the plan of the Soviet counterattack, and were thus both well prepared and nicely reinforced to repel the Soviet counterattack. By 1 August the 198^th Motorised Division was in full retreat. and the Finnish decision to order the II Armeijakunta to advance trapped the Soviet forces. By 7 August the 2^nd Divisioona of the II Armeijakunta had reached the shore of Lake Ladoga at Lahdenpohja, thus cutting the Soviet divisions to the north-west of Lake Ladoga from their intended lines of withdrawal. Near Sortavala the 2^nd Divisioona, 17^th Divisioana and 19^th Divisioona were reorganised into Eversti Einar Mäkinen’s new I Armeijakunta, and the town fell to the Finns on 15 August. The defending 168^th Division withdrew along the lake shore but were encircled, although the Soviets managed to evacuate most of their men across Lake Ladoga, but had to abandon large quantities of war matériel that was gratefully seized by the Finns.