Meanwhile, Syrian militants and allied rebels reportedly seized a key town from government forces in Hama

At least 33 people were killed in a US-led coalition strike on a school used as a centre for displaced people near a militant-held Syrian town, a monitor said on Wednesday. 

The Syrian Observatory for Human Rights said the strike south of Al-Mansoura, a town held by the Islamic State group in the northern province of Raqqa, "took place in the early hours of Tuesday." 

"We can now confirm that 33 people were killed, and they were displaced civilians from Raqqa, Aleppo and Homs," said Observatory head Rami Abdel Rahman. 

"They're still pulling bodies out of the rubble until now. Only two people were pulled out alive," Abdel Rahman told AFP. 

The British-based monitor relies on a network of sources inside Syria for its information.

The US-led coalition operating in Syria said it would investigate the claims, "since we have conducted several strikes near Raqqa".

Raqqa is Being Slaughtered Silently, an activist group that publishes news from IS-held territory in Syria, also reported the raids. 

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

The US-led coalition has been bombing IS in Syria since 2014 and is backing an offensive to defeat the group in Raqqa city, the de facto heart of the group's so-called "Islamic caliphate". 

Earlier this month, the coalition said its raids there and in Iraq had unintentionally killed at least 220 civilians.  

But other monitors say the number is much higher. 

Rebels advance in central Syria

Meanwhile, Syrian militants and allied rebels seized a key town from government forces on Tuesday in a new offensive in the central province of Hama, the SOHR and fighting factions said.

The surprise offensive was announced on Tuesday by the Tahrir al-Sham alliance, which is dominated by the Fateh al-Sham Front, once linked to Al-Qaeda.

"This afternoon, Tahrir al-Sham began preparing for military operations against (regime) army positions in Hama province," said Tahrir al-Sham press officer Imad al-Din Mujahid. 

Tahrir al-Sham published photos of a massive explosion that it said was a suicide attack against a government checkpoint inside Suran.

Hama province is of strategic importance to Assad, separating opposition forces in Idlib from Damascus to the south and the regime's coastal heartlands to the west.

"Tahrir al-Sham detonated two car bombs as they stormed the town of Suran and other villages," said Abdel Rahman.

The British-based monitor said the militants and rebel groups seized Suran and eight surrounding checkpoints within hours, despite heavy air strikes by government warplanes. 

The town has exchanged hands several times in Syria's six-year conflict, with government forces recapturing it in late October from opposition factions. 

The Free Idlib Army, which is taking part in the offensive, said it targeted the Hama military airport and other government positions on Tuesday. 

Rebel groups and Tahrir al-Sham were also waging an offensive in the Syrian capital on Tuesday in a bid to link two neighbourhoods they control in Damascus. 

Difficult peace talks 

The violence comes two days before UN-brokered peace talks are set to resume in Switzerland between government officials and opposition representatives.

The fifth round of Syrian peace talks aim to discuss issues of governance - a political transition - the constitution and elections, as well as counter-terrorism at the request of Damascus.

The United Nations said on Tuesday that all sides who had already attended the last round of negotiations in February had confirmed their attendance for this week.

"These will be 'proxy talks', direct talks have been excluded," a Western source close to the negotiations told AFP.

UN Syria envoy Staffan de Mistura, who wants the three main topics to be discussed "in parallel", will be in charge of mediating, the source said.

But his task will be tough as, according to analysts and diplomats, neither the opposition nor the government is likely to make concessions.

Syria's conflict erupted in March 2011 with protests against Assad's rule, but has evolved over the years into a complex civil war.

More than 320,000 people have been killed and millions more displaced by the conflict.