Six months after the ouster of longtime Syrian ruler Bashar al-Assad, the United States is steadily drawing down its presence as part of Operation Inherent Resolve (OIR), a military task force launched in 2014 to fight the Islamic State group (IS).

"The reduction of our OIR engagement on a military basis is happening," the US envoy for Syria, Tom Barrack, said in an interview with Turkey's NTV late on Monday.

"We've gone from eight bases to five to three. We'll eventually go to one."

But he admitted Syria still faced major security challenges under interim leader Ahmed al-Sharaa, whose Islamist-led coalition toppled Assad in December.

Assad's ouster brought an end to Syria's bloody 14-year civil war, but the new authorities have struggled to contain recent bouts of sectarian violence.

Barrack, who is also the US ambassador to Turkey, called for the "integration" of the country's ethnic and religious groups.

"It's very tribal still. It's very difficult to bring it together," he said.

But "I think that will happen," he added.

The Pentagon announced in April that the United States would halve its troops in Syria to less than 1,000 in the coming months, saying the IS presence had been reduced to "remnants".