Syrian President Bashar al-Assad said in a U.S. television interview that Islamic State of Iraq and Syria (ISIS), which has seized swaths of territory in Syria and Iraq, has been gaining recruits since the start of U.S.-led air strikes against the militant group.

Asked how much benefit he was getting from the strikes in Syria that began last September, Assad told CBS's 60 Minutes: "Sometimes you could have local benefit but in general if you want to talk in terms of ISIS, actually ISIS has expanded since the beginning of the strikes."

Assad, who has been fighting Islamist and other rebels since 2011, said in the interview aired on Sunday there were some estimates that ISIS was attracting 1,000 recruits a month in Syria.

"And Iraq, they are expanding in … in Libya and - many other - al-Qaeda affiliate organizations have announced their allegiance to ISIS. So that's the situation," Assad said, using another acronym for the militant group.

Washington is seeking a negotiated settlement to Syria's civil war that excludes Assad, but has made clear its top priority in Syria is the fight against ISIS militants.

Asked under what circumstances he would leave power, Assad said: "When I don't have the public support. When I don't represent the Syrian interests, and values."

In reply to a question about how he determined what support he had among Syrians, he said: "I don't determine. I sense. I feel. I'm in contact with them."