WASHINGTON — The US military said on Friday that its forces in Syria have completed a three-week operation to airlift several thousand Islamic State prisoners to Iraq as the American contingent continues to withdraw from Syria ahead of a potential confrontation with Iran.

US personnel transferred some 5,700 imprisoned adult male Islamic State fighters to Iraq in an operation that started on Jan. 21, US Central Command said early Friday. The final aircraft flight of detainees departed northeast Syria and landed in Iraq late on Thursday.

Why it matters: The evacuation of what the US military has deemed high-priority ISIS prisoners from Syria opens the door for the Pentagon to further withdraw its remaining troops from the country, a process military officials have described as "conditions-based."

The risky airlift operation, which involved busing thousands of ISIS prisoners via road convoys to a local airbase in Syria in coordination with government-aligned forces, was quietly carried out as CENTCOM leaders have been preparing for a potentially much bigger confrontation in the region.