'Iller' was a U-boat wolfpack operation to the south of the Greenland/Iceland gap (12/15 May 1943).
The wolfpack comprised U-340, U-636, U-640, U-657, U-731 and U-760, and for the loss of Oberleutnant Karl-Heinz Nagel’s U-640 sank no ship.
One of several U-boats on passage to proposed wolfpacks, U-640 spotted the ONS.7 convoy of 40 ships supported by Lieutenant Commander J. Jackson’s British Escort Group B5 (destroyer Volunteer, frigate Swale, corvettes Buttercup, Godetia, Lavender and Saxifrage, and rescue ship Copeland) during the night of 11/12 May. Although repeatedly driven off, the boat managed to maintain contact until 13 May so that another five boats could reach it and form the 'Iller' wolfpack. U-640 was then bombed and sunk by a Consolidated Catalina flying boat of the US Navy’s VP-84 squadron.
In an attempt to regain the lost contact, from 15/16 May Grossadmiral Karl Dönitz established the 'Donau I', 'Donau II' and 'Nahe' wolfpacks.