United Nations Secretary-General Antonio Guterres, addressing the Security Council on Syria, urged all states "to act responsibly in these dangerous circumstances."
"Increasing tensions and the inability to reach a compromise in the establishment of an accountability mechanism threaten to lead to a full-blown military escalation," Guterres told the 15-member council.
Russian UN ambassador told the security council that US, France, Britain are only interested in ousting Syrian government and containing Russia, adding that Russia has 'proof' Britain took part in staged Syria chemical attack.
Russia accused the West of having the sole aim in Syria of overthrowing the government as it urged US, French and British leaders to refrain from military action.
"We continue to observe dangerous military preparations for an illegal act of force against a sovereign state," Russian Ambassador Vassily Nebenzia told the Security Council.
US Ambassador Nikki Haley said Friday there should be no rush to launch military action on Syria, but that "at one point you have to do something."
Haley said US estimates Syrian president Assad has used chemical weapons in Syrian war at least 50 times, and that Russia has used the veto 12 times to protect Assad regime.
"You don't rush decisions like this," Haley told reporters, adding that if there is haste "you make a mistake."
Haley spoke ahead of a Security Council meeting called by Russia to discuss the threat of US-led military action in Syria.
President Donald Trump is analyzing all the information and taking measures to avoid any unwanted repercussions, she said.
"We have to know that there is proof and we have to know that we are taking every precaution necessary should we take action," she said.
Separate analysis by the United States, France and Britain have concluded that a chemical attack took place on Saturday in the rebel-held town of Douma, said Haley.
The United States is consulting with Britain and France about a joint military response to the attack in Douma that medics and rescuers said left at least 40 people dead on Saturday.
France warned Friday that Syria's government had reached a "point of no return" by using chemical weapons and vowed to confront the "intolerable threat" of such attacks to global security.
French Ambassador Francois Delattre told a Security Council meeting that world powers must respond to the threat of repeated use of toxic gas as a weapon in Syria's war.
By "deciding once again to sue chemical weapons" in the rebel-held town of Douma, "the regime has reached a point of no-return," Delattre said.
"France will shoulder its responsibility to end an intolerable threat to our collective security," he said. "We must cease the chemical weapons escalation in Syria."
The United States is consulting with Britain and France about a joint military response to the attack in Douma that medics and rescuers said killed at least 40 people Saturday.
Russia and Syria have denied that a chemical attack took place and said video footage and photographs were staged by President Bashar al-Assad's opponents.