'Sepals III' was a British operation by the Special Operations Executive with 'Sepals' to organise the despatch of 'Crofton', carry out sabotage activities, maintain contact with the Milorg military resistance organisation in German-occupied Norway, provide its groups with training and when possible supply them with weapons (February 1945).

The relevant three-man party of L. Schanche, B. Bjølseth and O. Mjeide, departed overland from Stockholm in neutral Sweden, after preparations had been started in January. Several acts of sabotage were undertaken from this base just inside the Swedish frontier.

From the summer of 1944, in 'Sepals', several bases were established along the border between Sweden and Norway, these including four on the Swedish side on the frontier, and these remained active to the end of the war in May in 1945. The object of this far-ranging effort was to strengthen the Norwegian resistance movement and help it in the final months of the war to retake parts of Norway, and was also intended to counter the influx of Soviet influence in the areas of northern Norway liberated by the Soviets without any Norwegian military involvement. In the purely intelligence aspect of the undertaking the agencies involved were the British Secret Intelligence Service, Norwegian XU resistance movement which initially co-operated with the Milorg but then distanced itself, and the Swedish armed forces' C-Byrå (C Agency).

In many cases the 'Sepals' bases were established in the mountain huts of the Swedish tourist association, and each base was manned by about 12 Norwegian special forces soldiers trained by the British Special Operations Executive for the Kompani Linge resistance movement and the 'police troop' camps on Eldberget in the Dala-Järna area. These Norwegian troops were provided with equipment and weapons by the US Office of Strategic Services, which sent the matériel sent to Sweden as diplomatic equipment and then transported to the bases on the Swedish side of the frontier by 1/19^th Jägarbataljon or by the Sami mountain herdsman Lasse Sarri. The best known of the men of the 1/19^th Jägarbataljon and 1/19 K (Norrbottens Regementes Jägarbataljon i Kiruna) involved in the undertaking were Stig Synnergren and Gösta Wetter Hall. The head of the C-Byrån Norr (C-Agency North) was Captain Fahle Iceberg, who was primarily responsible for the intelligence aspect of the operation, and he and his brother, Gunnnar Isberg, were couriers.

The most northerly 'Sepals' base was Base 1 'Mons' in the Pältsastuga area near Kattuvuoma, and there were also Base 2 'Anna' at Unna Allakas in Abisko mountains, Base III 'Truls' close to what is now the Silvervägen and, most importantly, Base IV 'Marit' and 'Lotte' near the eastern edge of Leinavatn and Vuoskajaure, just 3.1 miles (5 km) inside Sweden close to the Norwegian resistance forces' 'Kari' communications centre. The Norwegian part of the undertaking was headed by Håkon Kyllingmark, who had established the Norwegian network, and the US involvement was headed by Karl Gustav Tillström of the OSS’s Westfield Mission in Stockholm.

An especially important task for one of the bases was the transmission of weather reports, which were vital in the planning of bombing attacks against the battleship Tirpitz in the Altafjord close to Tromsø.