By Isabel Coles and Saif Hameed

NORTH OF MOSUL, Iraq/BAGHDAD (Reuters) – Iraqi troops entered Mosul from the north for the first time on Friday, part of a new phase in the battle for the city that also saw elite forces bridge a river under cover of darkness in an unprecedented night raid.

The operations were part of a major new push launched last week to seize ground in the city, after progress in the nearly three-month-old operation had stalled for weeks because of a need to slow the advance to protect civilians.

Troops would soon “cut the head of the snake” and drive the ultra-hardline group from its largest urban stronghold, Prime Minister Haidar al-Abadi said on Friday.

The battle for Mosul is the biggest ground operation in Iraq since the 2003 U.S.-led invasion. A victory by the 100,000-strong U.S.-backed pro-government force would probably spell the end for Islamic State’s self-styled caliphate that has ruled over millions of people in Iraq and Syria since 2014.

But the militants, who are thought to number several thousand in Mosul, continue to put up fierce resistance using suicide car bombs and snipers.

They carried out more attacks against security forces some 200 km south of Mosul on Friday, killing at least four soldiers, and are expected to pose a guerrilla threat to Iraq and Syria, and to plot attacks on the West, even if their caliphate falls.

A spokesman for Iraq’s elite Counter-Terrorism Service (CTS), which has taken the lead in much of the assault on the city, said troops had taken territory in an overnight raid across a Tigris River tributary in east Mosul.

“We used special equipment and had the element of surprise – the enemy did not expect us to mount a night offensive because all previous offensives were during the day,” Sabah al-Numan told Reuters.

Iraqi army units later breached the city from the north for the first time since the offensive began on Oct. 17, entering the residential al-Hadba apartments complex, officers at a nearby command post told Reuters. It was not immediately clear how much of the area they controlled amid resistance from Islamic State.

Iraqi forces have so far recaptured more than half of eastern Mosul, but they have yet to cross the Tigris to face insurgents who are still firmly in control of the western half of the city.

More than 100,000 civilians have fled, but 1.5 million people have stayed behind in the city, which commanders say forced the government troops to slow their advance.

The new phase in the battle has put U.S. troops in a more visible role than at any point since they withdrew from the country in 2011. President Barack Obama, who pulled all U.S. forces out of the country, has sent thousands back as advisers since Islamic State swept through the north in 2014.

In the latest phase of the operation, U.S. forces deploying more extensively in support of the Iraqi army, federal police and CTS can now be seen very close to the front lines.

U.S. forces located south of Mosul fired HIMARS vehicle-mounted rockets at Islamic State targets in a northern district on Friday.

The commander of the U.S.-led coalition backing Iraqi troops said this week the army and security forces had recently improved their coordination and were gaining momentum after advances had slowed in some areas in the first two months.

Islamic State forces swept into control of a third of Iraq when the army abandoned its positions and fled two years ago. But the Iraqi government says its security forces have since been rebuilt and have proven themselves in battles to recapture the lost ground. Prime Minister Abadi praised the Iraqi army on the anniversary of its establishment.

“The Iraqi army today has combat experience it has won in the war against terrorism … and is achieving victories and is clearing cities and villages,” he said in a statement.

‘HEAD OF THE SNAKE’

“The fight against terrorism is in its final round. Our forces … will cut off the head of the snake and clear all of Mosul soon, with God’s help,” Abadi said.

Abadi initially pledged the northern city would be retaken by the end of 2016, but after the offensive slowed he said last month it would take three more months to drive Islamic State from Iraq.

The fighters retreated from villages and towns around Mosul in the early stages of the campaign, but put up fierce resistance inside the city itself, deploying suicide car bombs and snipers, and avoiding retaliation by hiding among civilians, a tactic Baghdad says amounts to using them as human shields.

One resident in the recently recaptured Mithaq district said life was slowly beginning to return, with traders selling food and other supplies on the streets, but that mortar shells fired by Islamic State militants were falling at an increasing rate in the area, and killed several people on Thursday.

Although most Mosul residents have stayed in the city, more than 2,000 a day are fleeing, according to the United Nations, some heading for increasingly crowded camps in the surrounding countryside.

Islamic State has meanwhile launched attacks elsewhere in the country in what could be a taste of the tactics it will resort to once it loses Mosul.

Militants attacked an Iraqi army outpost and a police station near the city of Tikrit on Friday, killing at least four soldiers and wounding 12 others, military and police sources said.

The militants used a car bomb and two suicide attackers in their assault shortly after midnight on the army outpost in the town of al-Dour on Tikrit’s outskirts, killing two officers and two soldiers, the sources said.

Gunmen separately attacked the police station a short distance away and set fire to the building before fleeing the area. There were no casualties from that attack, the sources said.

A series of bomb attacks in Baghdad in recent days, some claimed by Islamic State, have killed scores of people.

Iraqi forces meanwhile pushed IS militants back in an area northwest of Baghdad towards the Syrian border, military sources said, in a separate, smaller operation launched on Thursday and part of the wider fight against the group in Anbar province.

They encountered little resistance from the militants, who withdrew towards Aneh, around 50 km northwest of the town of Haditha, the sources said.

(Reporting by Isabel Coles, Saif Hameed and Stephen Kalin; writing by John Davison; editing by Peter Graff)