The United States and Turkey will begin training and equipping thousands of moderate Syrian rebel forces on Sunday as part of a deal the two NATO allies signed last week, an official said.

"I can say that the train-and-equip (program) will begin as of March 1," Tanju Bilgic, spokesman for the Turkish foreign ministry, was quoted as saying by the state-run Anatolia news agency.

After several months of negotiations, Ankara and Washington signed an agreement on February 19 to train and arm Syria's anti-regime fighters.

Turkey, an outspoken critic of Syrian President Bashar Assad, hopes rebel factions to be trained will battle the regime in Damascus as well as insurgents from the Islamic State (IS) group who have seized large swathes of territory in Iraq and Syria right up to the Turkish border.

The U.S. government hopes the first trained rebel forces will be operational by the end of the year.

The goal is to train more than 5,000 Syrians in the first year of the program, and a total of 15,000 over a three-year period.

Turkey's hesitation to take a robust action against IS militants in the U.S.-led coalition has led to strains in its ties with the United States.

U.S. intelligence chief James Clapper said on Thursday that Turkey did not place a high priority on fighting IS jihadists and as a result foreign fighters had been able to travel through the country into Syria.

Turkey has long been under pressure to do more to thwart the transit of jihadists across its territory to the war-ravaged country.

But Turkish officials have insisted that they have done all they can to tighten border security and say Western states should provide Ankara with real time intelligence and a full list of suspects.