Turkey and the United States have reached a “principle agreement” to provide air support for some Syrian rebel forces, in a move that could signal growing involvement from Washington in the conflict, a Turkish official said.

Turkish Foreign Minister Mevlut Cavusoglu told the Daily Sabah on Monday that up to 15,000 moderate Syrian opposition forces that are currently being equipped and trained in Turkey would be provided further support in Syria.

"They have to be supported via air. If you do not protect them or provide air support, what is the point?," Cavusoglu told the newspaper. "There is a principle agreement on providing air support. How it is going to be provided is in the responsibility of the army."

Washington could not be reached for comment, Reuters reported.

The U.S.-led program to train and equip the moderate opposition to the Islamic State group has faced significant delays and difficulties. The Syrian government has slammed the program, claiming Washington is providing direct support for terrorists, while some rebels have dismissed the program as ineffective or criticized its focus on fighting ISIS instead of Bashar Assad's regime.

Turkey has long said that any support for the rebels would need to include taking direct action against Assad's regime as well, whereas Washington has maintained that Assad would need to leave as part of an agreed-upon political transition.

Although the U.S.-led coalition has initiated a bombing campaign against ISIS in Syria since last October, Washington has been wary of providing a “safe zone” for rebel groups to operate out of, for fear of being drawn into direct confrontation with Damascus.

Cavusoglu added that while fighting ISIS is the current priority, the “regime must also be stopped.”