US-led coalition strikes reportedly killed more than 40 civilians within 48 hours in northern Syria, as the Pentagon on Wednesday announced reinforcements to allies battling the Islamic State group in Raqa.

In the deadliest raid, a suspected coalition air raid hit a school being used as a temporary shelter for displaced families between IS's main stronghold in Raqa city and Tabqa, a key town it controls further west.

The US-led coalition has been carrying out air strikes against IS in Syria since 2014 and on Wednesday upped the ante with airlifts and fire support for allied forces on the ground, the Pentagon said.

Senior diplomats from the 68-member alliance met in Washington on Wednesday to hear details of US President Donald Trump's promised tough new strategy to eradicate the jihadist group in Iraq and Syria.

US Secretary of State Rex Tillerson said IS leader Abu Bakr al-Baghdadi would inevitably be killed as coalition and local forces continue to pile pressure on the jihadists.

"Nearly all of Abu Bakr Baghdadi's deputies are now dead, including the mastermind behind the attacks in Brussels, Paris and elsewhere," Tillerson said. "It is only a matter of time before Baghdadi himself meets this same fate."

As they gathered, the Syrian Observatory for Human Rights monitoring group said a coalition air strike early Tuesday had killed 33 people near the town of Al-Mansura, about 30 kilometres (20 miles) west of Raqa. 

"They were displaced civilians from Raqa, Aleppo and Homs," said Observatory head Rami Abdel Rahman. "Only two people were pulled out alive."

The coalition said it had carried out 18 strikes near Raqa on Tuesday, and a Pentagon spokesman said the alliance would investigate the reported deaths.

"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.

Later, another US-led air strike on a bakery and other shops killed at least eight people and wounded 15 at Tabqa on Wednesday afternoon, the Observatory said.

More people were still unaccounted for and likely trapped under rubble, it said.

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

- Airlifts, artillery -

In addition to its aerial sorties, the US has several hundred troops on the ground in Syria supporting the anti-IS offensive by the Syrian Democratic Forces, an alliance of Kurdish and Arab fighters.

An SDF commander said US troops had been airlifted in to back the battle for Tabqa, where senior IS commanders are based and where the group's Western hostages were once held.

"US forces and fighters from the Syrian Democratic Forces were helicoptered to 15 kilometres west of Tabqa," on the southern bank of the Euphrates River, an SDF commander told AFP.

Allied SDF fighters also crossed in boats from their positions on the northern bank of the river to meet up with the US forces on the southern bank.

A US defence official said Wednesday, under condition of anonymity, that US artillery was being used in the operation to seize the Tabqa dam.

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".

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 jihadists on another.

Control of war-ravaged Syria is divided between myriad armed groups -- rebels, jihadists, Kurdish militia and Syrian government forces.

- 'Concerns' ahead of Geneva -

Years of diplomatic efforts have failed to end Syria's raging six-year conflict, which began with protests against Assad's regime in 2011 but has since killed 320,000 people. 

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

The monitor said the rebels had advanced to four kilometres outside Hama city.

An AFP reporter said families were fleeing fighting there, their belongings piled in pick-up trucks.

Aid group Save the Children said at least 10,000 people had fled their homes in the Hama area in the past 24 hours.

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.

Source: AFP