Refugees from Syria's long civil war were making their way home on Wednesday, as a new interim prime minister said he had been appointed with the backing of the rebels who toppled President Bashar al-Assad.

U.S. officials, engaging with rebels led by Hayat Tahrir al-Sham (HTS), urged them not to assume automatic leadership of the country but instead run an inclusive process to form a transitional government.

The new government must "uphold clear commitments to fully respect the rights of minorities, facilitate the flow of humanitarian assistance to all in need, prevent Syria from being used as a base for terrorism or posing a threat to its neighbours," U.S. Secretary of State Antony Blinken said in a statement.

HTS is a former al Qaeda affiliate that led the anti-Assad revolt and has lately downplayed its jihadist roots.

In a brief address on state television on Tuesday, Mohammed al-Bashir, a figure little known across most of Syria, said he would lead the interim authority until March 1.

"Today we held a cabinet meeting that included a team from the Salvation government that was working in Idlib and its vicinity, and the government of the ousted regime," he said.

Bashir ran the rebel-led Salvation Government before the 12-day lightning rebel offensive swept into Damascus.

Behind him were two flags - the green, black and white flag flown by opponents of Assad throughout the civil war, and a white flag with the Islamic oath of faith in black writing, typically flown in Syria by Sunni Islamist fighters.