In a phone call on Wednesday, Admiral Brad Cooper urged Syrian President Ahmed al-Sharaa to "avoid any actions that could interfere" with the process.

The United States said Wednesday it was launching an operation to move up to 7,000 Islamic State group jihadists across the border.

The announcement came after the withdrawal of Kurdish-led forces who had long secured the detention camps, as Syrian government troops took control of Kurdish territory in the north and northeast.

Iraqi and US authorities have already announced the transfer of a first group of 150 prisoners, a mix of Iraqis and foreigners.

Sharaa and Cooper also discussed "the importance of Syrian government forces adhering to a ceasefire" with the Kurdish forces, the statement by the US military's Central Command added.

Syria's government announced a ceasefire on Tuesday night with the Syrian Democratic Forces (SDF), who have withdrawn from swaths of the north and northeast under military pressure.

The ceasefire is largely holding, though both sides have accused each other of violations.