Leaders of the Islamic State group are leaving their Syrian stronghold of Raqa, fleeing in the face of the Arab-Kurd offensive backed by the international coalition, the US Defense Department said Friday.

"We are starting to see now that a lot of senior ISIS leaders, a lot of their bureaucrats... are beginning the process of leaving Raqa," said Pentagon spokesman Navy Captain Jeff Davis, using an alternate acronym for the jihadist group.

"They have definitely taken note of the fact that the end is near in Raqa," he told reporters, describing a retreat that seems "very organized, orderly."

After a string of major losses in both Iraq and Syria, the jihadists' two main strongholds of Mosul and Raqa are both under attack from forces backed by a US-led coalition.

After a massive, four-month campaign, Iraqi forces are tightening the noose on Mosul, while in Syria, the Arab-Kurd alliance, the Syrian Democratic Forces, has been advancing to cut off IS access in Raqa.

According to the Pentagon spokesman, that objective is nearly complete, with the Islamic State group now controlling only one road, in the southeast of the city.

Located along the north bank of the Euphrates River, the road links Raqa to Deir Ezzor, said Davis, who did not give precise details about the extent of the jihadists' retreat.

Roads in the north and west are blocked by the Syrian Democratic Forces, and in the south, by the destruction of bridges on the Euphrates, he said.

While the isolation operations on Raqa have made progress, the coalition has not announced its plan for recapturing the city.

Still unknown is what role the armed wing of the Kurdish Democratic Union Party, the YPG, will play in the US-backed Arab-Kurd alliance.

The YPG, considered a terrorist group by Turkey, fights the Islamic State group in northern Syria as well as Ankara's forces.

Turkish President Recep Tayyip Erdogan has proposed that the coalition replace the YPG with Turkish forces.

The new US defense secretary, James Mattis, met Thursday with his Turkish counterpart Fikri Isik in Brussels.

Chairman of the US Joint Chiefs of Staff Joe Dunford is due to meet with the top Turkish military leader Friday in Turkey, the Pentagon said.