Damascus (AFP) - Syrian troops have found a mass grave containing the bodies of 42 people executed by Islamic State jihadists in Palmyra, as Washington warned the group's leader will eventually "taste justice".

IS has in recent months claimed responsibility for deadly attacks in Brussels and Paris, but has lost ground in Syria and Iraq.

Days after Syrian troops backed by Russian forces recaptured Palmyra and its ancient ruins, the army "uncovered a mass grave of officers, soldiers, members of the popular committees (pro-regime militia) and their relatives," a military source told AFP on Saturday.

Twenty-four of the victims were civilians, including three children, he said, asking not to be named.

"They were executed either by beheading or by shooting."

In a major symbolic and strategic coup for President Bashar al-Assad, the Syrian army last Sunday recaptured Palmyra and its UNESCO World Heritage Site, which IS had overrun in May 2015.

During their nearly 10-month occupation of Palmyra, the jihadists executed at least 280 people, according to the Syrian Observatory for Human Rights, a Britain-based monitor which confirmed the discovery of the mass grave.

Soon after IS stormed Palmyra, it shot dead 25 soldiers in the ancient Roman theatre.

It later released a video of the mass killing in which the executioners appeared to be children or teenagers.

Syria's five-year war has left at least 270,000 people dead. Few mass graves have been found, however.

Nearly a week on, few of Palmyra's up to 70,000 residents have returned.

- US mulls more special forces -

"People fear reprisal by the regime, and also the mines planted all over the city by IS," Observatory director Rami Abdel Rahman told AFP.

"In addition, many houses were flattened by Russian air strikes before Palmyra was reclaimed," he added.

About 70 kilometres (45 miles) to the west, Syrian troops on Saturday pounded the IS-held city of Sukhna, which the army wants to take back in order to consolidate its grip over Palmyra.

Story continues

"If the regime takes Sukhna, it will use it as a launching pad for an operation against Deir Ezzor province," in eastern Syria, along the Iraqi border, which is mostly controlled by IS, Abdel Rahman said.

The Syrian army has previously said the takeover of Palmyra would allow it to extend operations against IS in the east and around Raqa, the jihadists' de facto capital in the north.

At least 40 mostly foreign IS members, including 18 child soldiers, were killed in raids Thursday on a village in Deir Ezzor province, the Observatory said.

It was one of the single highest tolls that IS has suffered in a single strike since it emerged in Syria in 2013, the monitor said.

A US official on Saturday said the Pentagon was mulling over the possibility of sending more special forces to join the anti-IS fight in Syria, where the contingent of American fighters current numbers around 50.

"Presumably they would do more of what they're already doing," said the official, who declined to be named.

- Baghdadi will 'taste justice' -

IS has lost a string of high-ranking commanders in the past few weeks, mainly to strikes by the US-led coalition which launched a campaign against the jihadists in 2014.

On Wednesday, a drone strike near Raqa, likely by the US-led coalition, killed Abu al-Haija, a Tunisian commander summoned by IS leader Abu Bakr al-Baghdadi from Iraq.

On Friday, the Pentagon warned that Baghdadi himself, who in 2014 appointed himself "caliph" of swathes of Iraq and Syria, would eventually be hunted down and killed.

"Just like we found his mentor, (Abu Musab) al-Zarqawi and killed him. Just like we found the grand master of terrorism, Osama bin Laden, we killed him. We are going to find Baghdadi, and he will taste justice," military spokesman Colonel Steve Warren said.

While the truce has largely held, there have been reports of violence. The Observatory said regime air strikes on a rebel-held town east of Damascus Thursday left 33 dead, including 12 children.

Qatar condemned the raids with its foreign ministry warning that Syrian air strikes could "torpedo" the fragile ceasefire.

An unnamed Saudi foreign ministry source added in a statement that the kingdom condemns "in the strongest terms" the "ugly massacre by forces of Bashar al-Assad, the criminal."

In Aleppo province, Al-Qaeda's Syrian affiliate Al-Nusra Front and allied rebels seized the town of Al-Eis which overlooks a road linking second city Aleppo to Damascus, said the Observatory.