An officer in the elite Counter Terrorism Service, which has spearheaded the Mosul offensive, said troops had launched a major operation against the militants who are now almost surrounded in their last major urban redoubt in Iraq.

CTS special forces took over Malayeen, Samah, Khadra, Karkukli, Quds and Karama districts, the army said.

"This is something very big - it means large parts of the left bank have been liberated," CTS commander Lieutenant-General Talib Shaghati said, referring to the half of Mosul which lies on the east bank of the Tigris.

However, a resident of one district which the army declared recaptured told Reuters after the announcement that clashes continued.

"It's true urban warfare," he said by telephone.