MOSUL - Iraqi forces advanced in west Mosul and fighters in Syria seized a key supply route to Raqa on Monday as twin US-backed offensives gained ground against the Islamic State group.

Supported by the US-led anti-IS coalition, Iraqi government forces and a Kurdish-Arab alliance in Syria are battling to push the jihadists from Mosul and Raqa, the last two major urban centres under their control.

Intense fighting in recent days has forced tens of thousands of people from their homes, raising fears for many more civilians still trapped in areas of IS's so-called "caliphate". In Iraq, security forces advanced Monday towards a compound of jihadist-held government buildings and a bridgehead, on the second day of a renewed push in west Mosul.

The operation to retake west Mosul began on February 19, but had slowed amid several days of bad weather until a fresh drive began on Sunday.

A senior Iraqi commander said in a statement that interior ministry forces - backed by artillery and air support - were advancing in three neighbourhoods of west Mosul with the aim of taking a government compound held by the jihadists.

AFP reporters in west Mosul have witnessed intense clashes, with heavy automatic weapons fire and clouds of black smoke billowing over the city.

Lieutenant Colonel Abdulamir Mohammedawi of the interior ministry's elite Rapid Response Division told AFP its forces were "advancing in the Al-Dawasa and Al-Danadan areas to liberate the government buildings and secure a route for families to leave". Its sights were also set on the nearby Al-Hurriyah Bridge, he said, but Rapid Response had "not yet" reached it.

Mosul is divided by the Tigris River, and while the series of bridges crossing it have either been damaged or destroyed, they would provide a link between the Iraqi government-held east and IS-held west Mosul if they can be repaired or otherwise bridged.

Iraqi forces recaptured the western side of the fourth bridge, which is south of Al-Hurriyah, at the end of last month.

Iraq's Joint Operations Command also announced Monday that the elite Counter-Terrorism Service (CTS) had recaptured Al-Sumood, an area it had targeted as part of the push launched the day before.

The fighting in west Mosul has sparked an exodus from that side of the city, pushing more than 50,000 people to flee, according to the International Organization for Migration.

The operation to retake Mosul was launched on October 17, with a number of forces taking part but CTS and Rapid Response ultimately playing the leading roles in the advance.

In Syria, US-backed forces on Monday cut off a key supply route between IS stronghold Raqa and the group's territory in Deir Ezzor province.

The Syrian Democratic Forces (SDF), an alliance of Kurdish and Arab fighters, seized control of the only major road linking Raqa along the Euphrates valley to Deir Ezzor, the Syrian Observatory for Human Rights monitoring group said. "The route... linking Raqa to Deir Ezzor was cut this morning," an SDF commander confirmed.

The SDF launched its offensive for Raqa in early November and has since seized swathes of territory in northern Syria from the jihadists.

Its forces made a major incursion into the oil-rich Deir Ezzor province last month, in a drive to encircle and besiege the jihadists in Raqa.

Deir Ezzor province lies just east of Raqa and is almost completely held by IS. The jihadists also hold most of the provincial capital of the same name, and have been battling regime forces to overrun the city.

The SDF alliance, dominated by the Kurdish People's Protection Units (YPG), has benefitted from air support, equipment and training provided by the US-led coalition that has been carrying out air strikes against IS in Syria and neighbouring Iraq.

IS is under pressure on multiple fronts, not only Mosul and Raqa, but in other parts of northern Syria where it is facing offensives from government forces and Turkish-backed rebels.

Around 66,000 people have fled the fighting in Syria's north in recent days, UN humanitarian agency OCHA said.

IS, which emerged from the chaos of Syria's civil war to seize large parts of the country and neighbouring Iraq in 2014, has lost much of the territory it once claimed.