The United States and Turkey have agreed “in principle” to give air support to some forces from Syria’s mainstream opposition, Turkey’s foreign minister said, in what if confirmed could mark an expansion of U.S. involvement in the conflict.

There was no immediate comment from U.S. officials on the assertion — though Washington has so far refrained from committing to enforcing a “safe zone” for Syrian rebels, as it could be seen as a declaration of war on the Syrian state.

The air support would protect Syrian rebel forces who have been trained by a U.S.-led programme on Turkish territory, said minister Mevlut Cavusoglu. The long-delayed scheme is meant to send 15,000 troops back to Syria to fight Islamic State militants.

Cavusoglu did not go into details on what “in principle” meant or what kind of air power would be provided or by whom.

“They have to be supported via air. If you do not protect them or provide air support, what is the point?,” Cavusoglu told the pro-government Daily Sabah newspaper 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.”

The U.S-led training programme has been mired in delays amid media speculation of disagreements between the two NATO allies.

Turkey has said that any support programme must be part of a comprehensive strategy which includes battling forces loyal to Syrian President Bashar al-Assad.

Washington has maintained its opposition to Assad but said that the goal of training is only to defeat Islamic State militants.

Cavusoglu reiterated that while fighting Islamic State is prioritised, the “regime must also be stopped”.