BEIRUT - A US-led coalition strike is reported to have killed 33 civilians in northern Syria on Wednesday.

The Pentagon said the US military is providing air and artillery support to allied forces near a key town west of Raqa, IS's main Syrian stronghold, ahead of a major offensive against the militants.

A suspected coalition strike in the northern province of Raqa early Tuesday has killed 33 civilians, according to the Syrian Observatory for Human Rights.

The monitor said Wednesday that the strike hit a school being used as a temporary shelter for displaced families, about 30 kilometres (20 miles) west of Raqa.

"We can now confirm that 33 people were killed, and they were displaced civilians from Raqa, Aleppo and Homs," said Observatory head Rami Abdel Rahman. "Only two people were pulled out alive."

A Pentagon spokesman on Wednesday said the coalition would investigate the alleged strike.

"Since we have conducted several strikes near Raqa we will provide this information to our civilian casualty team for further investigation," he said. "Raqa is Being Slaughtered Silently," an activist group that publishes news from IS-held territory in Syria, blamed the coalition for the strike.

"The school that was targeted hosts nearly 50 displaced families," it said.

Earlier this month, the coalition said its campaign in Syria and Iraq had unintentionally killed at least 220 civilians, but monitors say the real number is far higher. 

IS is under pressure from several directions in northern Syria, with Russia supporting its Syrian ally President Bashar al-Assad on one front and Turkey providing air cover for rebel groups battling the militants on another.

In addition to its aerial sorties, the US has several hundred troops on the ground in Syria supporting anti-IS advances.

A US defence official said Wednesday, under condition of anonymity, that US artillery was being used in an operation to seize a strategic dam around 30 kilometres (18 miles) west of Raqa.

Another official, Pentagon spokesman Adrian Rankine-Galloway, said "coalition forces are assisting... with airlift and fire support in an operation to seize the Tabqa dam." Control of war-ravaged Syria is divided between myriad armed groups - rebels, militants, Kurdish militia and Syrian government forces.

This week, rebels and allied militants launched two surprise offensives on government positions in Damascus and central Hama province, opposition groups and the Observatory said.

In Damascus, anti-regime factions are battling to link up territory they hold in the eastern district of Jobar with their encircled forces in the Qabun neighbourhood to the north. Clashes continued Wednesday as at least nine air strikes pounded rebel positions, the Observatory said. Damascus has witnessed heavy fighting since Sunday, when the opposition fighters made their largest incursion yet into the capital's east.

On Wednesday, AFP correspondents in the city could hear warplanes buzzing overhead and intermittent mortar fire, as several soldiers were seen loading ammunition into machine guns.

Damascus resident Abu Rita said he had been trapped at home by fighting for four days.

"I left for the first time today" the 62-year-old told AFP. "They were the hardest four days of my life. The sound of shelling didn't stop and one of my windows was shattered."

Years of diplomatic efforts have failed to end Syria's raging six-year conflict, which began with protests against Assad's regime.

The war has killed more than 320,000 people, sparked a major refugee crisis, and dragged in world powers including the US-led air coalition bombing Syria's militants since 2014.

Rebels and militants launched a fresh assault on Tuesday in the central province of Hama, overrunning the key town of Suran and several other government positions.

Footage published by the Tahrir al-Sham rebel alliance depicted fighters firing rockets from a field onto regime positions.

The escalating violence comes just a day before a new round of UN-brokered peace talks in Geneva hosted by UN special envoy Staffan de Mistura.

In Moscow on Wednesday for final meetings before the talks, de Mistura said the developments "raise concerns".

"We must seek to achieve a political process as quickly as possible," he said.

De Mistura will travel to rebel backer Ankara on Thursday and will then return to Switzerland to lead the talks.