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The United States has expanded a runway at a northern Syria air base to accommodate the large C-17 military plane needed to help with logistics for the push to retake the IS stronghold of Raqqa, a US defence official said Tuesday.

The huge military air freighter will play an essential role in supplying the Syrian Democratic Forces as they mount an offensive to retake the city, IS's last stronghold in Syria.

The US military has in recent weeks sent hundreds of extra troops into Syria, including a Marine artillery contingent that is targeting Raqqa.

The airstrip near the Kurdish town of Kobane was first used by a C-17 in December, the military's Central Command spokesman Colonel John Thomas said.

"It's a very important logistical hub for the Raqqa operation," Thomas told AFP, in public comments confirming a story that first appeared in the Stars and Stripes military newspaper.

The C-17 can carry "all of the small armoured technical vehicles and small armoured infantry vehicles that are in play and authorised to move into the region," he added.

Thomas said the improved air base, located about 90 miles (145 kilometres) north of Raqqa, is not dissimilar from another base the US helped expand near Mosul in Iraq ahead of the fight for that city.

"Air lift and logistics are important for supporting our partner forces on the ground – this is the kind of capability that is helping a lot," Thomas said.

The SDF launched its offensive for the city of Raqqa in November, seizing around two-thirds of the surrounding province, according to the pro-opposition Britain-based Syrian Observatory for Human Rights.

At their closest point, they are just eight kilometres (five miles) from the city, to the northeast.

The Pentagon is arming and training the SDF, but questions remain over how much support the United States should give the Kurdish component of the alliance, given concerns from Turkey, which views the Kurdish fighters as "terrorists."