MOSUL, Iraq (Reuters) - The bombed out, battered streets, homes and buildings around Mosul museum were eerily quiet, other than the hum from the engines of Iraqi forces’ armored vehicles stationed outside.

A tank of Iraqi rapid response forces is seen during clashes with Islamic State militants in Mosul, Iraq March 13, 2017. Picture taken March 13, 2017. REUTERS/Azad Lashkari

The usual sound of planes and helicopter gunships attacking Islamic State positions in the city was also absent, with cloud cover and rain making any air raids difficult to coordinate.

These are the conditions Islamic State fighters holed up in the city center relish.

“They use the cloud and the rain to launch attacks, and it stops any advances by us,” said Sergeant Ali Abu Hamra of the Rapid Response, an elite interior ministry force.

Sure enough, as units moved up towards the front line, mortar and gunfire began coming in, crashing down around the museum which Iraqi forces seized last week.

Troops took cover in a utilities room annexed to the museum complex, while other units ran forward to shoot at Islamic State positions and try to suppress incoming gunfire.

Monday’s weather was not a game-changer. It was simply a boost to the stiff resistance of Islamic State fighters who have retreated into more crowded urban areas including Mosul’s old city, making life difficult for the Rapid Response force.

The deserted museum and nearby government complex were recaptured almost a week ago, but the area, where federal police have filled in as elite raiding parties try to move forward, is far from secure.

The struggle, rain or shine, to make significant advances beyond this district which borders the old city limits, shows how tough the fight for Mosul’s heart is likely to be as jihadists hail shell and sniper fire from inside.

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Islamic State have held out in Mosul, their last major urban stronghold in Iraq, far longer than the government initially predicted as it launched a U.S.-backed campaign in October to drive the militants out.

Iraqi forces in January brought all districts east of the Tigris river under their control, and last month launched attacks in western Mosul, which houses the Nineveh provincial government headquarters and the old city with its narrow streets.

The old city contains the mosque from which Islamic State leader Abu-Bakr al-Baghdadi declared himself caliph over large areas of northern Iraq and eastern Syria in 2014.

The Federal Police and Rapid Response said at the weekend they had entered the old city.

But there was no significant progress as the week began, and the militants, firing from inside, pinned down Iraqi forces for hours.

Rapid Response troops operating around the museum last week had already said it was hard for them to consolidate gains from raids without areas behind them fully secured.

ALEPPO STREET

Wounded troops sat or lay on mattresses in the dark room next to the museum, making sure the metal door was closed to stop shrapnel flying in.

“The fight is in the old city now, but it’s tough, the streets are narrow,” said Nasser Taleb, 34, a bandage over his eye from a bullet ricochet wound he said he suffered on Aleppo Street on the edge of the old city, where intense fighting took place at the weekend.

The fighting is set to cost many more lives and wreak further destruction as it moves into the heart of the city. Near the museum, air strikes had pounded deep holes in the roads and churned up tarmac.

Palm trees along a central boulevard were blackened, their trunks gashed open by explosions.

As the morning’s fighting wore on and the rain came down, Iraqi artillery and rocket launchers positioned further back began to pound targets in or around the old city. Planes could be heard in the skies again, but did not launch many strikes.

A soldier at a base away from the frontline asked: “Who’s advancing today, us or them?”