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.

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.

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.

But the ultra-hardline 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.

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 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.

ISIS militants 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.