US sanctions against Iran over the Islamic republic's suspect nuclear program are biting, Senior Democratic Senator John Kerry said Wednesday in the wake of talks in Syria, a key Tehran ally.

"They clearly are having some impact, and I think that folks there are perhaps a little surprised at the amount of impact," said Kerry, who met this week in Damascus with Syrian President Bashar al-Assad.

"They don't think it's the way to solve the problem," and "they believe that you gotta go the other way," through negotiations, said the lawmaker, who chairs the Senate Foreign Relations Committee.

But Kerry said he had warned Damascus that was "very difficult" because of Iranian President Mahmoud Ahmadinejad's past approach to negotiations and offers from major world powers to freeze his country's atomic drive.

Tehran denies the West's charges that it masks a covert quest for nuclear weapons.