"I can tell you today that the United States will deploy approximately 200 additional US forces in Syria," the Pentagon chief told Gulf policymakers in the Bahraini capital.

The two cities are the last major urban centres under IS control after the jihadists suffered a string of territorial losses in both countries over the past year.

The operation coincides with a vast US-backed offensive to retake Iraq's second city of Mosul from the jihadists, forming a twin-pronged campaign intended to deliver a knockout blow to the "caliphate" they declared across Iraq and Syria in 2014.

They will join 300 US special forces troops already deployed in support of the Syrian Democratic Forces alliance in the drive on the jihadist stronghold it launched on November 5.

Washington will send another 200 troops tohostility to the SDF of Turkey, a NATO ally and Syria's neighbour.

Ankara regards the alliance's most powerful military component, the Kurdish People's Protection Units (YPG), as an arm of the outlawed rebel Kurdistan Workers Party (PKK), which has waged a deadly insurgency in southeastern Turkey for three decades.

The alliance controls a large swathe of northeastern Syria along the Turkish border as well as a smaller enclave in the northwest.

US Defence Secretary Ashton Carter addresses a press conference during a visit to Kabul, on December 9, 2016 (photo by: NOORULLAH SHIRZADA/AFP)

After a series of advances that looked set to join up the two areas of control, the Turkish army entered Syria in August in an operation it said was aimed at both IS and the YPG.

Turkish troops have since