Turkey and the U.S. have agreed in "principle" to provide air protection to Syrian rebels being trained and equipped to fight Islamic State militants, once they enter Syrian territory for battle, Turkey's foreign minister said.

The two countries agreed in February to train and equip up to 15,000 Syrians under the $500 million U.S. program designed to add a credible ground force to an air bombing campaign against the militant group.

The program suffered unexplained delays as the U.S. resisted Turkish calls for the force to be also prepared to fight the Syrian regime and the sides tried to reach agreement on who to train.

The U.S. has also refused a Turkish proposal for enforcing a safe area and a no-fly zone in Syria, prompting questions on how trained rebels would be protected when they entered Syria.

"They have to be supported via air," Mevlut Cavusoglu, the Turkish foreign minister told the pro-government Daily Sabah newspaper in comments published Monday. "If you do not protect them or provide air support, what is the point?"

Cavusoglu didn't provide further details on the air cover or confirm whether the air support would include the use of armed drones taking off from an air base in southern Turkey.

"These are technical details," Cavusoglu told Daily Sabah during a visit to Seoul. "There is a principle agreement on providing air support. How it is going to be provided is in the responsibility of the army."

U.S. officials have said the aim is to train 5,000 Syrians a year for three years at a base in the central Turkish city of Kirsehir and at sites in Saudi Arabia and Qatar.

While resisting Turkish pressure for the rebels to also target Syrian regime forces, U.S. officials have conceded that the recruits will have to defend themselves against all sides.

Cavusoglu said: "While the fight against (the Islamic State group) is prioritized, the regime must be also stopped."