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Dozens of US Special Operations Forces have been deployed to Syria's border with Turkey to fight the Islamic State group there, in support of the Turkish military and "vetted" Syrian rebels, the Pentagon said Friday.

Last month, Ankara launched an ambitious military offensive in northern Syria dubbed "Euphrates Shield", sending in tanks and special forces to support opposition fighters in a bid to remove IS fighters and Kurdish militia forces from its frontier.

"At the request of the government of Turkey, US special operations forces are accompanying Turkish and vetted Syrian opposition forces as they continue to clear territory from ISIL in and around the area of the Syrian border near Jarabulus and Al-Rai," said Pentagon spokesman Jeff Davis using an alternate name for IS.

That area is near the flashpoint city of Aleppo.

A US defence official said there were several dozen troops in the new special forces contingent.

The chairman of the Joint Chiefs of Staff, General Joseph Dunford, discussed the fight against IS earlier with his Turkish counterpart General Hulusi Akar on the sidelines of a NATO chiefs of staff meeting in Croatia.

"General Dunford met with General Akar to advance discussions on the way forward in the fight against ISIL, and recommitted to the close military-to-military and strategic relationship the US has with Turkey," said Captain Gregory Hicks, a special assistant to the chairman of the Joint Chiefs of Staff.

A fragile truce in Syria announced early this month appears to be holding, despite growing frustration over aid deliveries being held up at the Turkish border.

The special forces deployment comes as Washington accused Damascus of blocking aid to besieged cities and warned it will not boost military cooperation with Russia unless Moscow is able to convince President Bashar al-Assad both to halt attacks on rebel groups and allow humanitarian access.