US-backed forces pressed offensives Wednesday on the Islamic State group's strongholds in Syria and Iraq, as a strike by the American-led coalition was reported to have killed 20 civilians near Raqa.

Supported by coalition air raids, Iraqi forces have pushed into IS's Mosul stronghold and a Kurdish-Arab militia alliance has been advancing on the jihadists' de facto Syrian capital Raqa.

The Syrian Observatory for Human Rights said a coalition strike overnight had hit the IS-held village of Al-Heisha, about 40 kilometres (25 miles) north of Raqa.

Rami Abdul Rahman, the head of the Britain-based monitoring group, said nine women and two children were among the 20 dead civilians and that 32 others had been wounded.

"There is no such thing, and any such claims are IS news," SDF spokeswoman Jihan Sheikh Ahmed told AFP. The US-led coalition said it was checking on the report.

The SDF's online media account said at least six IS fighters had been killed by coalition strikes in the village and accused the extremist group of preventing civilians from leaving Al-Heisha in a bid to use them as "human shields".

'We left everything behind'

Civilians fleeing Al-Heisha told AFP on Tuesday that IS fighters had brought heavy weaponry into the village when the SDF operation began.

"Daesh fighters brought heavy weapons to our village and stayed among us so that if there were strikes they would hit us," 45-year-old Saada al-Aboud said, using an Arabic acronym for IS.

"They wouldn't let us leave. We had to escape by running out into the fields, with our children and old people. What else could we do? We left everything behind."

The SDF launched the drive towards Raqa on Saturday, upping pressure on the jihadists three weeks after Iraqi forces began their assault on Mosul.

Raqa and Mosul are the last major cities in Syria and Iraq under IS control, after the jihadists suffered losses that greatly reduced the size of the self-styled "caliphate" they declared in mid-2014 following their seizure of large parts of both countries.

The US-led coalition, which launched the air campaign against IS two years ago, hopes that driving the group from the two cities will deal it a knockout blow.

The SDF has been pushing south from areas near the Turkish border towards Raqa, seizing a series of villages and moving to positions about 35 kilometres (22 miles) from the city.

In Iraq, an official said Tuesday that Kurdish peshmerga fighters had seized the town of Bashiqa near Mosul, which would be a final step in securing the eastern approaches to Mosul.

42,000 forced from homes

There were still pockets of resistance in the town, officials said, and an AFP correspondent in Bashiqa reported continued air strikes, gunfire and explosions on Tuesday.

The peshmerga and Iraqi federal forces have been advancing on Mosul from the north, east and south since October 17, and last week pushed into the city's eastern outskirts.

Officials are warning of long and bloody battles ahead in both Mosul and Raqa, with IS expected to put up fierce resistance and use trapped civilians as human shields.

More than a million people are believed to be in Mosul. Raqa in 2011 had a pre-war population of some 240,000, and more than 80,000 people have since fled there from elsewhere in Syria.

The International Organization for Migration said Wednesday that nearly 42,000 people had fled their homes since the start of the Mosul operation.

That was an increase of more than 7,000 from the figure the IOM gave the previous day, but it was unclear when the spike in displacement occurred.

jpegMpeg4-1280x720Fleeing civilians have described living under the brutal rule of IS, which has committed widespread atrocities, from stonings and beheadings to the trading of sex slaves.

After seizing control of the town of Hamam al-Alil earlier this week, Iraqi forces reported the discovery of 100 decapitated bodies in a mass grave in the area.

Iraqi forensics experts were investigating the site, where an AFP journalist saw bodies and bones among piles of rubbish.