Over 400 US Marines involved in battling the Islamic State group in Syria are being withdrawn as part of a cut in forces after the capture of jihadist "capital" Raqa, the US-led coalition said Thursday."The Marines supported Coalition partner forces with precise artillery strikes in the fight against ISIS," the coalition said in a statement.

"With the city liberated and ISIS on the run, the unit has been ordered home. Its replacements have been called off."

The coalition's director of operations Brigadier General Jonathan Braga called the move "a real sign of progress" as the jihadists have seen the vast swathes of ground they seized across Syria and Iraq in 2014 reduced to just a few remaining pockets.

"We're drawing down combat forces where it makes sense, but still continuing our efforts to help Syrian and Iraqi partners maintain security," he said in the statement.

A recent report from the Pentagon's Defense Manpower Data Center said that as of September 30 the US military had 1,720 troops in Syria and 8,892 in Iraq.

Those numbers were far above the officially released figure of 503 in Syria and 5,262 in Iraq.

An alliance of Kurdish and Arab fighters, the Syrian Democratic Forces (SDF), retook Raqa from the jihadists in October after a brutal onslaught supported by artillery and air power from the US-led coalition.