A US-led coalition has carried out some of its heaviest air strikes yet on the Islamic State group's de facto Syrian capital, killing more than 30 people, including six civilians.

The strikes on Saturday night and Sunday morning also damaged infrastructure in Raqa city, the group's bastion in northern Syria.

Elsewhere, regime forces backed by Lebanon's Hezbollah advanced on the last rebel-held town in the Qalamun region by the Lebanese border.

And fighting continued between government troops and two rebel coalitions seeking to capture territory from the regime in northern Aleppo city.

In Raqa, the Syrian Observatory for Human Rights said at least 30 people had been killed in US-led coalition strikes late Saturday and early Sunday.

The dead included six civilians, among them a child, but the rest were IS fighters, the Britain-based monitor said.

The US-led coalition said the strikes were some of its heaviest since it began carrying out raids against IS in Syria last September.

"The significant air strikes tonight were executed to deny Daesh (IS) the ability to move military capabilities throughout Syria and into Iraq," spokesman Lieutenant Colonel Thomas Gilleran said in a coalition statement.

"This was one of the largest deliberate engagements we have conducted to date in Syria and it will have debilitating effects on Daesh's ability to move from Raqa."

Coalition forces "successfully engaged multiple targets" throughout Raqa, the statement said, destroying IS structures and transit routes.

The strikes "have severely constricted terrorist freedom of movement," it added.

- IS executions in Palmyra -

The raids came after IS released a video Saturday showing the execution of 25 Syrian soldiers in the ancient amphitheatre in the city of Palmyra.

The executions had been reported earlier, in the days after IS seized the town from government forces on May 21, but the video was the first evidence of the killings.

The soldiers were shot in the head by boys and teenagers in military uniforms, with a large IS flag hung behind them on the amphitheatre's stage.

Palmyra's ancient ruins are listed as a UNESCO World Heritage Site and there has been concern that IS might seek to destroy the city's heritage, as it has done elsewhere in Syria and Iraq.

In Syria's Qalamun region, regime forces backed by fighters from the Lebanese Hezbollah movement advanced against rebels in an operation to capture the town of Zabadani.

The town was one of the first to fall to the opposition in 2012, and is now the last major town in the Qalamun area near Damascus that remains in rebel hands.

The Observatory said at least 14 regime forces and Hezbollah fighters had been killed in fighting for the town in the last 24 hours, along with at least 11 rebels.

The monitor said regime forces had pounded the city and its surroundings with dozens of aerial attacks, as well as rocket fire and heavy artillery in the last 48 hours.

In Aleppo, fighting continued between government forces and two rebel coalitions seeking to capture regime-held districts in the west of the city.

The Observatory said an assault by a grouping of Islamist fighters and Al-Qaeda affiliate Al-Nusra Front on the Zahra neighbourhood had largely failed, but that fighting on the outskirts of the district was ongoing.

A second coalition, known as Conquest of Aleppo and grouping more moderate rebels, has captured a military barracks in one neighbourhood, though regime forces were battling Sunday to recapture the facility.

Aleppo city, once Syria's economic powerhouse, has been divided into government-held territory in the west and rebel-held territory in the east since shortly after fighting arrived there in mid-2012.

The situation is largely reversed in the countryside surrounding the city, and both sides have at times attempted to encircle the other portion of Aleppo and lay siege to their opponents.

The latest fighting in western Aleppo began earlier this week and is some of the fiercest seen in the area since mid-2012.

More than 230,000 people have been killed in Syria's conflict, which began in March 2011.