MOSUL, IRAQ : Advancing Iraqi troops broke through Islamic State defenses in an eastern suburb of Mosul on Monday, taking the battle for the insurgents' stronghold into the city limits for the first time, a force commander said.

The fighting came after two weeks of advances by U.S.-backed Iraqi forces who cleared surrounding areas of insurgents, in the early stages of the largest military operation in Iraq since the invasion that toppled Saddam Hussein in 2003.

Commanders have said the battle for the city, the hardline militants' last big bastion in Iraq, could take months. Troops of the Iraqi army's Counter-Terrorism Service (CTS) moved forward on Gogjali, an industrial zone on the eastern outskirts.

The commander of CTS forces east of the city, Lieutenant-General Abdul Ghani al-Assadi, told state television his forces had reached the edge of the Karama district inside the city.

A Reuters correspondent in the village of Bazwaia saw plumes of smoke rising from a built-up area a few kilometers away which a commander said was the result of clashes already under way inside Karama.

A Kurdish peshmerga intelligence source said he received a report saying seven Islamic State militants were killed in the Aden district, adjacent to Karama, and two of their vehicles destroyed.

Iraqi state television said there were also clashes inside the city between Islamic State fighters and residents rising up against the group.

The Kurdish intelligence source said such "resistance elements" had opened fire on an Islamic State police unit in Intisaar district, south of Karama, and armed fighters had spread out in streets across the city apparently fearing revolt.

Reuters could not independently verify the report. The government and its U.S. allies are hoping an uprising inside the city will help loosen the grip of the fighters, who seized it in 2014 and proclaimed a "caliphate" to rule over all Muslims.

The fighting ahead in a built-up city still home to 1.5 million people will be more complex than the recent capture of Christian and Sunni Muslim villages and towns outside the city, mostly emptied of their residents.