US Ambassador Nikki Haley on Monday urged the United Nations Security Council to act following the latest alleged chemical weapons attack in Syria, warning the United States was ready to respond.
Russia said a US military strike against Syria would have "grave repercussions" and stressed that the use of chlorine or sarin had not been confirmed in the attack Saturday in Douma.
"We have reached the moment when the world must see justice done," Haley told an emergency meeting at UN headquarters in New York.
Britain, France, the United States and six other countries requested the urgent meeting after toxic gas was allegedly used in the rebel-held town of Douma, killing at least 40 people.
"History will record this as the moment when the Security Council either discharged its duty or demonstrated its utter and complete failure to protect the people of Syria," she said.
"Either way, the United States will respond."
Russian Ambassador Vassily Nebenzia said Moscow had told the United States that it would not allow its forces on the ground in Syria to be put at risk.
"Armed force under mendacious pretext against Syria, where, at the request of the legitimate government of a country, Russian troops have been deployed, could lead to grave repercussions," Nebenzia said.
Russian experts on the ground have found no evidence of sarin or chlorine use, said the ambassador, who offered Syrian and Russian assistance to allow investigators from the Organisation for the Prohibition of Chemical Weapons to travel to the site.
Nebenzia accused Western powers of pursuing a "confrontational policy" using "slander, insults, hawkish rhetoric, blackmail, sanctions and threats to use force."
Speaking in Washington, President Donald Trump vowed earlier that "major decisions" would be made in the "next 24-48 hours" as Defense Secretary Jim Mattis said he wasn't ruling out military action.
The United States is determined to "see that the monster who dropped chemical weapons on the Syrian people is held to account," Haley said.
Washington earlier presented a draft resolution that would establish a new independent inquiry of chemical attacks in Syria, but diplomats said the measure was unlikely to win Moscow's support.
In November, Russia used its veto power at the council to block the renewal of a previous UN-led probe that found the Syrian air force had dropped sarin on the town of Khan Sheikhun in April last year.
It was not known when the proposed US measure would come up for a vote at the council.
France accused Russia of having a hand in the attack on Douma that drew global outrage.
"These attacks took place either with Russia's tacit or explicit agreement, or despite its military presence on the ground," said French Ambassador Francois Delattre.