Rebel leader Ahmad al-Sharaa's Islamist group is stamping its authority on Syria's state with the same lightning speed that it seized the country, deploying police, installing an interim government and meeting foreign envoys - raising concerns over how inclusive Damascus' new rulers intend to be.

Since Sharaa's Hayat Tahrir al-Sham (HTS) group swept Bashar al-Assad from power on Sunday at the head of a rebel alliance, its bureaucrats - who until last week were running an Islamist administration in a remote corner of Syria's northwest - have moved into government headquarters in Damascus.

The appointment of Mohammed al-Bashir, the head of the regional government in HTS' enclave of Idlib, as Syria's new interim prime minister on Monday underlined the group's status as the most powerful of the armed groups that battled for more than 13 years to end Assad's iron-fisted rule.

Although it was part of al Qaeda before breaking ties in 2016, HTS had reassured tribal leaders, local officials, and ordinary Syrians during its march to Damascus that it would protect minority faiths, winning broad approval. The message helped smooth the rebels advance and Sharaa - better known as Abu Mohammed al-Golani - has repeated it since Assad's ouster.

At the office of the Damascus governor, its walls exquisitely decorated with marquetry and stained glass, the man brought from Idlib to run affairs dismissed concerns that Syria was being moved towards an Islamic form of government.

"There is no such thing as Islamic governance. After all, we are Muslims and it's civil institutions or ministries," said Mohammed Ghazal, a bespectacled 36-year-old civil engineer with a thick beard who was raised in the United Arab Emirates and spoke in near perfect English.

"We don't have any problem with any ethnicity and religion," he said. "The one who made the problem was the (Assad) regime."

However, the way HTS has gone about shaping the new interim government - by bringing senior administrators from Idlib - has caused concern for some. Four opposition sources and three diplomats told Reuters they were concerned about the inclusiveness of the process so far.

Bashir has said he will only remain in power until March. But HTS - which remains classified as a terrorist group by the United States, regional powerbroker Turkey and other governments - 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.

'RUINS, RUINS, RUINS'

Like other members of the HTS-affiliated Salvation Government in Idlib brought to Damascus to run state bodies, Ghazal said he had given assurances to employees and urged them to return to work. "It's a collapsed state. It's ruins, ruins, ruins," Ghazal said.

His priorities for the next three months are getting basic services running and streamlining the bureaucracy. Salaries, which average some $25 a month, would be increased in line with Salvation Government wages. Its minimum wage is $100 a month.

"Syria is a very rich country," said Ghazal, asked how this would be financed. "The regime used to steal the money."

Policemen brought from Idlib are directing traffic in Damascus, trying to restore some normalcy since HTS ordered armed groups out of the city. One officer, who did not give his name, said they were stretched thin, noting they previously just had to patrol Idlib.

Though HTS is preeminent among the factions which fought Assad, others remain armed, notably in areas at the borders with Jordan and Turkey.

During the war, rebel factions often clashed with each, leaving a legacy of rivalries and enmity seen as one of many risks to stability in post-Assad Syria.

Yezid Sayigh, a senior fellow at the Carnegie Middle East Center, said HTS "clearly is seeking to maintain the momentum on all levels", adding that any group in their position, taking over from a collapsed regime in an exhausted country, would behave broadly the same way.

"There are multiple risks with HTS setting priorities and the pace for what comes next. One of these is to establish a new form of authoritarian rule, this time in Islamic garb," he said.

But he assessed the diversity of Syria's opposition and society would make it difficult for one group to monopolize influence.

Turkey - an influential backer of the opposition - was also keen for a government that could win international backing, he said.

'WE WILL ONLY STAY UNTIL MARCH'

An opposition source familiar with HTS consultations said all of Syria's sects would have representation in a caretaker government. Issues to be determined in the next three months included whether Syria should have a presidential or parliamentary system of government, the source said.

The Syrian revolt spiralled out of the 2011 Arab Spring uprisings that toppled autocrats in Egypt, Tunisia, Libya and Yemen, giving rise to turbulent and often violent periods of transition.

In an interview with Italian newspaper Il Corriere della Sera published on Wednesday, Prime Minister Bashir said "we will only stay until March 2025".

The priorities, he said, were restoring security and state authority, bringing home millions of Syrian refugees, and providing essential services.

Asked whether Syria's new constitution would be Islamic, he said "these details" would be clarified in the constitution-making process.

Mohammed Alaa Ghanem, a leading Syrian activist based in Washington and in touch with senior opposition figures, said HTS was being urged to "be smart and get the transition right, instead of letting the moment go to their heads by completely dominating the new government".

The Biden administration has urged HTS not to assume automatic leadership of Syria but instead run an inclusive process to form a transitional government, according to two U.S. officials and a congressional aide briefed on the first U.S. contacts with the group.

U.S. Secretary of State Antony Blinken has said the transition in Syria should lead to "credible, inclusive, and non-sectarian governance" consistent with U.N. Security Council Resolution 2254.

That resolution, approved in 2015, calls for a Syrian-led process facilitated by the United Nations, establishing within six months non-sectarian governance and setting a schedule for a process of drafting a new constitution.

It also calls for free and fair elections.

One diplomat in Damascus said HTS is the only faction meeting with foreign missions. "We are concerned - where are all the heads of the political opposition," the diplomat said. "It would be a major signal to have them here, and they are not here."

A second diplomat said HTS had conveyed good messages to the public but the degree of inclusiveness displayed in recent days was disturbing. Constitutional reform, in particular, must be an inclusive process and would be a really big test.

The diplomat noted the presence of many other factions which have yet to disarm or demobilise as a potentially destabilising factor if an inclusive process does not take place.

Joshua Landis, a Syria expert and head of the Center for Middle East Studies at the University of Oklahoma, said Sharaa "must assert authority quickly to stop a decent into chaos".

"But he must also try to scale up his administrative capacity by bringing in technocrats and representatives of the diverse communities," he said.