At least 30 civilians, including 13 children, were killed in air strikes in eastern Syria on Friday, a monitoring group said.

It was unclear if the raids in Deir Ezzor province, which is mostly controlled by the Islamic State group, were carried out by Russian or regime warplanes, the Britain-based Syrian Observatory for Human Rights said.

A cousin of Syria's President Bashar Assad has been sentenced to 20 years in prison for the murder of a military officer last summer, a security source said Friday.

"A court in Latakia on Thursday sentenced Suleiman Assad to 20 years in prison for the murder of Colonel Hassan al-Sheikh," the source said.

The United Nations said Thursday that crucial talks on ending the Syria conflict would likely be delayed by a few days, as a dispute over who will represent the opposition intensified.

The talks, due to start in Geneva on Monday, are part of the biggest diplomatic push yet to resolve a civil war that in nearly five years has left more than 260,000 dead and forced millions from their homes.

U.S. Secretary of State John Kerry said Thursday peace talks to solve the crisis in Syria would proceed on January 25 as planned, despite a dispute over who would represent the country's opposition.

"It may be a day or two for invitations but there is not going to be a fundamental delay. The process will begin on the 25th and they will get together and see where we are," Kerry said on the sidelines of meetings at the World Economic Forum in Davos, Switzerland.

French President Francois Hollande said Thursday that a coalition waging a bombing campaign against the Islamic State group would "accelerate" air strikes.

"The pace of the interventions will be accelerated and France will play its role in this," Hollande said in a speech to ambassadors.

The U.S. and French defense ministers on Wednesday condemned Moscow's role in the Syria conflict, saying Russian jets should stop targeting the opposition forces fighting the Islamic State group.

"The Russians are on the wrong track strategically and also in some cases tactically," said U.S. Defense Secretary Ashton Carter after a meeting in Paris of seven defense ministers in the coalition fighting IS.

Hundreds of displaced Syrian families returned to check on their homes in southern Damascus on Wednesday under a truce deal between the regime and opposition fighters.

Carrying plastic bags and lugging suitcases, they gathered at a train station in the capital waiting to enter the rebel stronghold of Qadam.

Syria peace talks are expected to begin within a few days, Russian Foreign Minister Sergei Lavrov said Wednesday, adding that Moscow was ready to cooperate closer with the United States on Syria aid supplies.

Lavrov, who met his U.S. counterpart John Kerry in Zurich Wednesday in a bid to create momentum for Syria peace talks to kick off as planned on January 25, rejected suggestions the negotiations might be delayed until February amid disagreements over who will represent the opposition.

Iranian Foreign Minister Mohammad Javad Zarif said on Wednesday there could be no military solution to the devastating war in Syria and said further confrontation with Saudi Arabia was in no one's interest.

"There is no military solution to the (Syrian) crisis. We need a political solution," Zarif told an audience at the World Economic Forum in the Swiss resort of Davos.

A deadly Islamic State group assault on Deir Ezzor has seen the jihadists tighten their siege on the eastern Syrian city and left residents terrified and fearing the worst.

Fierce clashes have rocked the city's northern edges after a multi-front offensive on Saturday that cost the lives of dozens of people.