EAST OF MOSUL, Iraq (Reuters) - 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.

Mosul is many times larger than any other city Islamic State has held, and the United Nations has warned of a worst-case scenario of up to 1 million people being suddenly displaced, requiring the world’s largest humanitarian operation.

“SURRENDER OR DIE”

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Prime Minister Haider al-Abadi, speaking at the Qayyara military airbase south of Mosul, said the Iraqi forces were trying to close off all escape routes for the several thousand Islamic State fighters inside Mosul.

“God willing, we will chop off the snake’s head,” Abadi, wearing military fatigues, told state television. “They have no escape, they either die or surrender.”

Iraqi security forces and Kurdish peshmerga fighters started the offensive against the hardline Sunni group on Oct. 17, with air and ground support from a U.S.-led coalition.

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“They are making deliberate progress, they’re on their timeline,” British Major General Rupert Jones, deputy commander for strategy and support of the U.S.-led anti-Islamic State coalition, told Reuters.

The recapture of Mosul would mark the militants’ effective defeat in the Iraqi half of the territory they seized two years ago.

Ranged against them are some 50,000 Iraqi troops, policemen and Kurdish peshmerga, with air and ground support from the U.S.-led coalition. Thousands of battle-hardened Iran-backed Shi’ite militia fighters also joined the campaign west of the city two days ago.

Hadi al-Amiri, leader of the Badr Organisation, the largest of the Shi’ite militia groups, expressed hope that Mosul would not descend into a protracted and devastating conflict like the four-year-old battle for the Syrian city of Aleppo, where Shi’ite militias are also fighting.

“We are afraid that Mosul would be another Aleppo, but we hope that will not happen,” he told reporters in Zarqa, south of Mosul.

SCORCHED EARTH TACTICS

Islamic State militants have been fighting off the offensive with suicide car bombs, snipers and mortar fire.

Islamic State said on Monday it carried out a suicide operation against a joint convoy of the army and Shi’ite militias south of Mosul. It gave no casualty figures.

The militants have brought displaced thousands of civilians from villages toward Mosul, using them as “human shields” to cover their retreat, U.N. officials and villagers have said.

They have also set oil on fire to create smokescreens, choking the region in smoke.

“Scorched earth tactics employed by retreating ISIL members are having an immediate health impact on civilians, and risk long-term environmental and health consequences,” the U.N.’s Office for the Coordination of Humanitarian Affairs said.

The warring parties have given no casualty figures among their own ranks or civilians. Both say they have killed hundreds of their opponents.