Over 400 US Marines involved in battling the Islamic State group in Syria are being withdrawn as part of a cut in forces after the capture of jihadist "capital" Raqa, the US-led coalition said Thursday.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 had deployed to Syria in March and used 155mm howitzers to support local forces as they fought to retake Raqa.

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

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," Braga 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, and even after the announcement of the Marine withdrawal, that supposed 503 figure hadn't budged.

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.

The international group -- which began targeting the jihadists in 2014 -- is still supporting the militia in an operation in neighboring Deir Ezzor province.

The Syrian regime backed by Russian fire power is currently conducting a separate offensive in the same region.

As operations change from taking back territory to more of a counter-insurgency, the need for equipment like heavy artillery has diminished, Pentagon spokesman Major Adrian Rankine-Galloway said.

Now the goal is more to "train local security forces to defend their territory," he said.

- 'Adjustments to military support' -

Since Raqa's capture, coalition strikes have dropped off drastically and, under pressure from Turkey, Washington has promised "adjustments to the military support" for the SDF.

Kurdish officials have, however, insisted that cooperation will continue between the two sides, while the US has said coalition forces do not expect to end their mission until a peace deal is negotiated in Syria.

In a separate statement released Thursday the coalition said that 801 civilians had been killed unintentionally in the more than 28,000 air strikes it has conducted, and that it is still probing 695 reported incidents.

Monitoring group Airwars insists that number is well below the true civilian toll of the bombing campaign, estimating that almost 6,000 innocent lives have been lost.

As the "caliphate" the jihadists declared now looks set to be wiped off the map, there are major questions over what comes next in Syria's complex war that has claimed over 340,000 lives since 2011.

Syrian President Bashar al-Assad -- and his main backers Iran and Russia -- now have clear military superiority in the country and international players led by Moscow are making their most concerted push to halt hostilities.

But any progress towards an overall political settlement remains fraught with obstacles and UN-backed talks involving the regime and opposition currently taking place in Geneva have been clouded by major disagreements.