Russia called for the UN Security Council to convene in an emergency session Thursday amid tensions over a suspected chemical attack in Syria.

Vassily Nebenzia told reporters at the UN this is the exact time when the council should be meeting.

"The immediate priority is to avert the danger of war," Nebenzia said.

"The second priority now is for the OPCW mission to reach Damascus and Douma, and to see what's really happened," he added, referring to the Organization for the Prohibition of Chemical Weapons.

The meeting Russia is proposing is meant to address the possibility of potential U.S. military action in response to the alleged use of chemical weapons by the Bashar al-Assad regime.

UN sources told Anadolu Agency that Russia is seeking to hold the meeting Friday.

A team from the OPCW has been tasked with determining if a chemical attack took place but not determining responsibility.

It confirmed its team is on its way to Syria and expects to commence work Saturday.

The White Helmets, a civil defense agency, blamed the Assad regime for a chemical attack Saturday night in the city of Douma in Eastern Ghouta which it said killed 78 civilians and injured hundreds of others.

U.S. Defense Secretary James Mattis said Thursday that President Donald Trump has yet to decide on whether he will pursue military action in Syria.

"We have not yet made any decision to launch military attacks into Syria," Mattis told lawmakers on the House Armed Services committee. "The president has not made that decision."

Asked what worries him most about possible military action the U.S. might take, Mattis said: "On a strategic level, it's how do we keep this from escalating out of control."

Earlier Thursday, Trump appeared to walk back previous comments in which he warned Russia to brace for missiles which he said "will be coming".

"Never said when an attack on Syria would take place. Could be very soon or not so soon at all!" Trump said Thursday on Twitter.

Later, he told reporters a decision will "be made fairly soon".