The meeting was the first of its kind for Acting Defense Secretary Patrick Shanahan, who started his new job January 1 following the resignation of his predecessor Jim Mattis.

According to a senior defense official, Shanahan met with National Security Advisor John Bolton, Secretary of State Mike Pompeo and General Joe Dunford, chairman of the Joint Chiefs of Staff.

They "discussed a wide variety of topics to include Syria," the official said.

Mattis quit last month after Trump ordered the withdrawal of US troops from Syria, even though the Islamic State still has thousands of fighters in the country and a US pull out would leave Kurdish fighters vulnerable to attack by Turkey.

Shanahan must oversee how the US pulls more than 2,000 troops out of Syria.

Various timelines have been floated, but Trump on Wednesday seemed vague, saying only that it would happen "over a period of time."