* Forced recruitment sign of attrition
* Residents pour out of eastern Mosul
* Battle focused on Old City but IS hold other areas
By Patrick Markey and John Davison
MOSUL, Iraq, March 20 (Reuters) - Islamic State militants are forcing young men in western Mosul to fight for them in defence of the remaining pockets of their former stronghold against an Iraqi government assault, fleeing residents and Iraqi officers said on Monday.
The forced recruitment is a sign that the hardline militants are growing more desperate as the battle for what was once the de facto capital of their self-proclaimed Islamic caliphate enters its sixth month.
Elite Federal Police and Rapid Response units on Monday resumed their cautious advance on the al-Nuri Mosque in western Mosul's Old City. But thousands of people took advantage of the fog and rain in the early morning to flee Islamic State-controlled areas and reach the safety of government lines.
The militants were using residents as human shields, hiding in houses and forcing young men to fight, several refugees said.
Ali, a former government worker, said he had hidden his sons in a basement when Islamic State fighters came looking for recruits.
"It feels like the siege is ending. All they are doing now is defending," he told Reuters. "I hid my sons in the basement and told them if you want my sons you will have to kill me."
Yassin, a butcher who also escaped from the west side, said Islamic State held less ground than before.
"They would come to my butcher shop looking for people so people stayed away. People even stopped going to the mosque because sometimes they would come to take people from there to join the fighting," he said.
Residents who had left said Iraqi, Syrian and other foreign militants remaining in the area tried to make them stay.
"A French militant beat me and threatened me to force me to stay," said one woman who gave her name as Um Tahseen.
A Federal Police intelligence officer, Captain Ali al-Kinani, said the militants wanted to fill up their ranks as they had suffered heavy casualties.
Some fighters were wearing civilian clothes under their uniforms and would switch outfits to mix in with fleeing civilians, he said.
"We arrested dozens who said that they were forced by Daesh to carry arms or take a bullet in the head if they refused," Kinani said, using an Arab acronym for Islamic State.
"Many families that fled the fight asked our troops to help their sons. Some young men hiding inside their houses are still waiting for our forces to secure their neighbourhoods and rescue them."
If the militants' behaviour indicates that Islamic State is steadily being worn down, the battle for Mosul is still expected to last several more weeks. The government offensive was launched in October with support from U.S. artillery, air strikes and advisers and the eastern side of the city on the Tigris river was secured in December.
The campaign for the western side, now in its second month, is harder as it is more densely populated, the streets are narrower and the houses closely packed together.
Attention has focused on the Old City and the al-Nuri Mosque, from where Islamic State leader Abu Bakr al-Baghdadi proclaimed the caliphate after his forces had seized swathes of territory in Iraq and Syria in 2014.
Baghdadi and other IS leaders have fled Mosul, whereabouts unknown, but the militants still hold several other districts of western Mosul beyond the Old City and are defending them with sniper and mortar fire and suicide car bombs. U.S. officials estimate that about 2,000 IS fighters remain there.
At the same time, IS forces in the Syrian city of Raqqa are under attack in a parallel conflict.
On Sunday, a U.S.-led coalition air strike on an IS command centre in western Mosul killed six foreign militant commanders, including a Russian, Abdul Kareem al-Rusi, who headed the Tareq Bin Ziyad Brigade.
The number of displaced people from both sides of Mosul since the start of the offensive has reached 355,000, Iraq's immigration minister said on Monday. Some 181,000 had poured out of western Mosul since the start of the operations to retake that side a month ago, Minister Jassim Mohammed said.
The United Nations refugee agency is opening new camps to handle the exodus as the present ones are overflowing.
(Additional reporting by Ahmed Rasheed in Baghdad, Writing by Angus MacSwan in Erbil,; Editing by Ed Osmond)