Iraqi Prime Minister Haider al-Abadi declared victory in the “liberated” city of Mosul on Sunday in what his office called "the biggest defeat yet for the Islamic State (IS) group".

"The PM arrived in the liberated city of Mosul and congratulated the heroic fighters and the Iraqi people on the achievement of the major victory,” his office said in a statement.

The announcement comes after a gruelling battle that lasted nearly nine months to retake the northern city from the militants who had been controlling Mosul for three years.

A photo on Abadi's official Twitter account showed him dressed in a black military uniform and cap as he arrived in Mosul to announce the recapture of the city.

Gunfire still audible in Mosul

The fighting did not seem to be completely over yet, with gunfire still audible in Mosul and air strikes hitting the city around the time the premier's office released the statement.

The declared victory in Mosul marks an epic milestone for the Iraqi security forces, who had crumbled in the face of an IS onslaught across Iraq in 2014.

IS swept across much of Iraq's heartland in a lightning offensive that year, proclaiming a self-styled “caliphate” straddling Iraq and neighbouring Syria. But the militant group, which is facing twin offensives backed by a US-led coalition in both countries, has since lost large parts of the territory it once controlled.

In recent days, security forces have killed militants trying to escape their dwindling foothold in Mosul, as Iraqi units fought to retake the last two IS-held areas near the Tigris.

Earlier Sunday Iraq's Joint Operations Command had said it killed “30 terrorists” trying to escape across the river. Even in the final days of the battle, thousands of civilians remained trapped inside the Old City and those who fled arrived grief-stricken after losing relatives in sniper fire and bombardments.

Around 915,000 residents have fled Mosul since the start of the battle for the city in October, the United Nations said this week.

Iraqi forces are backed by air strikes and advisers of the US-led coalition fighting IS in Iraq and Syria since 2014.

Abadi declared on Twitter late last month that “we are seeing the end of the fake (IS) state”. The recapture of Mosul will not however mark the end of the threat posed by IS, which holds territory elsewhere in Iraq and is able to carry out frequent bombings in government-held areas.