Syria's new interim prime minister pledged to protect minority rights as Washington's top diplomat flew to the region on Thursday to urge an "inclusive" transition after the ouster of president Bashar al-Assad.

Mohammad al-Bashir has said he will only remain in power until March.

But the new coalition of rebel groups has yet to spell out key details of the transition process, including its thinking on a new constitution.

"You are bringing (ministers) from one colour, there should be participation of others," said Zakaria Malahifji, secretary general of the Syrian National Movement who once served as political advisor to rebels in Aleppo. He said the lack of consultation in forming an interim government was a misstep.

"Syrian society is diverse in terms of cultures, ethnicities, so frankly this is concerning," he said.

Assad fled Syria last Sunday after a lightning offensive spearheaded by the Islamist Hayat Tahrir al-Sham (HTS) group and its allies, which brought a spectacular end to five decades of rule by the Assad clan.

Syrians across the country and around the world erupted in celebration after enduring decades during which suspected dissidents were jailed or killed, and nearly 14 years of war that killed 500,000 people and displaced millions.

The joy sparked by Assad's overthrow has been accompanied by uncertainty about the future of the multi-ethnic, multi-confessional country.

Its new rulers have sought to reassure minorities they will be protected.

Sunni Muslim HTS is rooted in Syria's branch of Al-Qaeda and is proscribed as a terrorist organisation by many Western governments, though it has sought to moderate its rhetoric.

"Precisely because we are Islamic, we will guarantee the rights of all people and all sects in Syria," Mohammad al-Bashir, the transitional head of government until March 1, told Italian daily Corriere della Sera in an interview published on Wednesday.

The new rulers have also pledged justice for the victims of Assad's iron-fisted rule, with HTS leader Abu Mohammed al-Jolani vowing that officials involved in torturing detainees will not be pardoned.

Jolani, now using his real name Ahmed al-Sharaa, also urged "countries to hand over any of those criminals who may have fled so they can be brought to justice".

Top US diplomat Antony Blinken is on Thursday in Jordan, his first stop on a regional tour that will see him offer American support for "an inclusive, Syrian-led transition to an accountable and representative government," State Department spokesman Matthew Miller said.

He will call for Syria to avoid becoming a "base of terrorism" or "posing a threat to its neighbours" -- a nod to the concerns of Turkey and Israel, both of which have carried out strikes inside Syria since Assad's fall. (Agencies)