The United States and Turkey have signed an agreement to train and equip moderate Syrian opposition fighters, a Turkish Foreign Ministry official said on Thursday.

The US military has said it is planning to send more than 400 troops, including special operations forces, to train Syrian moderates at sites outside Syria as part of the fight against Islamic State militants.

"The agreement was signed by the foreign ministry undersecretary and the US ambassador," the official told Reuters.

US officials have said they plan to train about 5,000 Syrian fighters a year for three years under the plan. Saudi Arabia and Qatar, as well as Turkey, have publicly offered to host training sites.

Turkey hopes the training will also bolster the weakened and divided Syrian opposition in their war against Syrian President Bashar al-Assad.

The Free Syrian Army is seen by Turkey as a key actor in Syria's conflict but the group has been riven by divisions and suffered setbacks at the hands of government forces and other rebel factions.