Syria's deputy foreign minister says air support provided by any country to rebels fighting against Islamic State extremists would be considered an "aggression."

Faisal Mekdad's comments on Tuesday were in response to Turkish Foreign Minister Mevlut Cavusoglu, who said Turkey and the U.S. have agreed in "principle" to provide air protection to rebels being trained and equipped to fight Islamic State militants, once they enter Syrian territory for battle.

Ankara and Washington agreed in February to train and equip up to 15,000 Syrian fighters under a U.S. program designed to add a credible ground force to an air bombing campaign against the militant group.

Cavusoglu didn't provide further details on the air cover.

Mekdad made the comments to the Lebanon-based Al-Mayadeen TV station.