"Any interested party in Syria should understand that attacking US Forces or our coalition partners will be a bad policy," Lieutenant General Kenneth McKenzie, director of the joint staff, said during a press conference.

His remarks came after Assad told Russian broadcaster RT he would not hesitate to use force to retake the third of the country held by the Syrian Democratic Forces.

"The only problem left in Syria is the SDF," he told the channel, adding he saw "two options."

"The first one: we started now opening doors for negotiations. Because the majority of them are Syrians, supposedly they like their country, they don't like to be puppets to any foreigners," Assad said in English.

"We have one option, to live with each other as Syrians. If not, we're going to resort... to liberating those areas by force."

Pentagon spokeswoman Dana White later clarified the US did not want to get involved in Syria's civil war, and would offer protection only in areas where the SDF are fighting the Islamic State group.

The SDF, dominated by the militia of a self-proclaimed Kurdish autonomous administration, has air support from the US-led coalition against IS and backing from US and French special forces on the ground.

Both the SDF and Russian-backed Syrian troops are engaged in separate operations against IS in east Syria, creating a highly volatile situation, where de-confliction mechanisms have already been tested several times.

The SDF has clashed with Syrian regime fighters on the ground, and the coalition has bombed government forces and their allies on multiple occasions.