BEIRUT/BAGHDAD - 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. 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.

Meanwhile, Islamic State suicide bombers and fighters attacked the centre of Iraq’s northern oil refinery town of Baiji overnight, forcing the army and fighters to pull back, military sources and the local mayor said on Sunday.

The town of Baiji and its refinery - Iraq’s largest - have been a battlefront for more than a year. The hardline Islamists seized the town in June 2014 as they swept through much of northern Iraq towards the capital Baghdad.

Control of Baiji neighbourhoods has changed hands many times during the conflict. The latest Islamic State offensive comes after authorities said they controlled nearly the whole town and expected to drive insurgents from the refinery within days.

The militants attacked around 8 pm (1700 GMT) on Saturday with two suicide car bombings. The blasts were followed by fierce clashes that lasted until midnight and drove the army and mainly Hashd Shaabi forces from the centre of town, two army colonels said.

Baiji mayor Mahmoud al-Jabouri said there had been a pattern of withdrawals by Islamic State fighters in the town followed by counter-offensives. “Their lethal weapons are suicide attacks and snipers, and this is why we have fighting back and forth.”

Army officers said the army and Hashd groups were preparing a response. “Islamic State fighters are still holding positions in three neighbourhoods in Baiji and they are still receiving reinforcements,” said one of the army colonels.

In Anbar province west of Baghdad, witnesses said two rockets hit a crowd in the Islamic State-controlled provincial capital Ramadi on Saturday evening, killing at least 18 people.

They said a group of people had gathered after the daily Ramadan fast to play Muhaibis, a game where players have to identify a member of the opposing team who is hiding a ring.

“I heard a blast and saw fire coming from Dolphin Square. I ran to the place and saw vehicles carrying bodies and wounded covered with blood. They were innocent people playing a ring game; they were not making bombs,” said Haj Thamir Ahmed, a Ramadi resident who lives nearby.

In northwest Baghdad, at least three people were killed and 11 wounded when a bomb went off near a restaurant in the mainly Shia district of Shulaa on Sunday morning, police and medical sources said. Another two people were killed by a bomb in Hussainiya on the city’s northern outskirts.

There was no immediate claim of responsibility for those attacks, but statements in the name of Islamic State said the group carried car bombings on Saturday evening in Baghdad and Balad Roz which killed 10 people.