ERBIL, Iraq, Dec 29 (Reuters) - Iraqi security forces launched a fresh advance on Thursday against Islamic State militants in several southeastern districts of Mosul, where the fight had been stalled for about a month, Interior Ministry officials said.

"Our troops now are advancing. In the first five or 10 minutes they took 500 metres. Just now they are starting to shoot," said an officer from the rapid response forces, an elite Interior Ministry unit.

Those forces were advancing in Intisar district, while thousands of federal police troops redeployed from Mosul's southern outskirts two weeks ago were expected to push into a nearby area, he said.

The battle for Mosul, involving 100,000 Iraqi troops, members of the Kurdish security forces and Shi'ite militiamen, is the biggest ground operation in Iraq since the U.S.-led invasion of 2003.

Elite Iraqi soldiers have retaken a quarter of Mosul, the militants' last major stronghold in Iraq, but their advance has been slow and punishing. They entered a planned "operational refit" earlier this month, the first significant pause of the campaign.

The upcoming phase appears likely to give U.S. military advisers, part of an international coalition fighting Islamic State, a bigger role as they embed more extensively with Iraqi forces.

Mosul, the largest city held by Islamic State anywhere across its once vast territorial holdings in Iraq and neighbouring Syria, has been held by the group since its fighters drove the U.S.-trained Iraqi army out in June 2014.

Its fall would probably end the group's ambition to rule over millions of people in a self-styled caliphate, but the fighters could still mount a traditional insurgency in Iraq, and plot or inspire attacks on the West.

Iraqi Prime Minister Haider al-Abadi, who previously pledged to retake Mosul by the end of the year, said this week it would take another three months to rout Islamic State in Iraq.

(Reporting By Stephen Kalin; Editing by Paul Tait)

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