US ambassador to the United Nations, Nikki Haley, said on Wednesday that Syria's President Bashar al-Assad is a "big hindrance in trying to move forward" to find an end to the country's six-year conflict.

"I'm not going to go back into should Assad be in or out, been there, done that, right, in terms of what the US has done," she told the Council of Foreign Relations. "But I will tell you that he is a big hindrance in trying to move forward, Iran is a big hindrance in trying to move forward."

With Russian and Iranian military support, Assad has the upper hand in a war with rebels who have been trying to topple him with backing from states including Turkey, Saudi Arabia and the United States. A US-led coalition has also been targeting Islamic State militants in Syria.

"This is one of those situations where the US and Russia could definitely talk and say 'OK, how can we get to a better solution.' But the issue of Assad is going to be there," Haley said.

UN-led peace talks are currently being held in Geneva. Haley said UN mediator Staffan de Mistura "desperately" wants the United States to be part of finding a solution for the conflict in Syria.

"When you have a leader who will go so far as use chemical weapons on their own people you have to wonder if that's somebody you can even work with," she said. Assad's government has denied using chemical weapons.

"If we don't have a stable Syria, we don't have a stable region and its only going to get worse. It really is an international threat right now and we have got to find a solution to it," Haley said.