President Bashar al-Assad has said U.S.-led air strikes against Islamic State militants in Syria should be subject to an agreement with Damascus and Syrian troops should be involved on the ground.
Assad was speaking in an interview with the U.S.-based Foreign Affairs Magazine published on Monday.
"With any country that is serious about fighting terrorism, we are ready to make cooperation, if they're serious," Assad said, when asked if he would be willing to take steps to make cooperation easier with Washington.
Washington supports opposition forces fighting for the past four years to topple Assad, but its position has become complicated since Islamic State and other hardline groups emerged as the most powerful rebel factions.
Since Islamic State took over much of Syria and Iraq last summer, the United States has mounted regular air strikes against it. But it has rejected the idea of allying itself with the Syrian government despite them now having a common enemy.
When asked what he would like to see from the United States, Assad said Washington should pressure Turkey not to allow money and weapons into northern Syriaand "to make legal cooperation with Syria and start by asking permission from our government to make such attacks".
"The format we can discuss later, but you start with permission. Is it an agreement? Is it a treaty? That's another issue," he said.
Washington informed Damascus before it started strikes in Syria in September.
The power of the hardline Islamists, including Islamic State and the al Qaeda-affiliated Nusra Front, makes it more difficult for the United States to find a suitable ally on the ground.
It plans to train and equip members of the mainstream Syrian opposition to fight Islamic State as part of its strategy to roll back the group's gains in Syria.
A first group of about 100 U.S. troops will head to the Middle East in the next few days to establish training sites for Syrian opposition fighters.
Assad said the campaign should be backed up by the Syrian army on the ground.
"The question you have to ask the Americans is, which troops are you going to depend on? Definitely, it has to be Syrian troops."
The United Nations says 200,000 people have been killed in the civil war, which started with pro-democracy protests that were violently repressed.
