The United States threatened Syria with military action as President Donald Trump warned "something should happen" following a suspected chemical attack that left at least 86 dead and provoked global outrage.
Meanwhile, Russia warned America of potential "negative consequences" if Washington strikes Syria.
"All responsibility if military action occurs will be on the shoulders of those who initiated such a doubtful tragic enterprise," Russian Ambassador to the UN Vladimir Safronkov said.
The sudden specter of US military action against Syrian President Bashar al-Assad's regime marks a startling about-face for Trump, and a crucial shift in American involvement in Syria's grueling six-year civil war.
US Secretary of State Rex Tillerson had earlier vowed an "appropriate response" to the attack in Khan Sheikhun in rebel-held Idlib province.
Twenty-seven children were among the dead, and Turkey believes victims were exposed to the nerve agent sarin.
As the Pentagon readied military options, the UN Security Council met behind closed doors to try to bridge differences over a draft resolution demanding an investigation of the suspected chemical attack.
Britain, France and the United States had pushed for a vote on their proposed measure, but decided to hold off during the meeting.
Diplomats said a vote was more likely to take place on Friday.
The fast-moving events come just days after the Trump administration had signaled it was no longer looking to ouster the Syrian leader.
But on Thursday, Trump said that "what Assad did is terrible."
"What happened in Syria is truly one of the egregious crimes," he said. "I guess he's running things, so I guess something should happen."
Trump's comments came as Tillerson -- who like the president was in Florida to welcome China's Xi Jinping -- called for "a political process that would lead to Assad leaving" and said his future role in the country was "uncertain."
The US ambassador to the UN, Nikki Haley, has warned that Washington could take unilateral action if the world body fails to respond to the allegations of chemical weapons use.
A US official said the Pentagon is presenting the White House with a range of possible options, including cruise missile or air strikes on Assad's air fields in a bid to ground his air force -- but that no decisions had been taken.
Any strike against the Assad regime is likely to be conducted remotely or by unmanned aircraft to avoid putting US pilots in harm's way.
The US Navy's Sixth Fleet has two destroyers -- the USS Porter and the USS Ross -- in the eastern Mediterranean that could shoot a barrage of Tomahawk cruise missiles into Syria.
Any such military action brings enormous risks, as strikes could be subject to skirting Russian air defenses. Moscow also has advisors on the ground in Syria.
- Syrian denial -
Trump on Wednesday decried the attack as an "affront to humanity." He seemed horrified by photographs showing dead children and victims suffering convulsions, breathing problems and foaming at the mouth.
Results from post-mortems point to possible exposure to sarin nerve agent, according to Turkish health officials.
"It crossed a lot of lines for me," Trump said, alluding to Barack Obama's failure to enforce his own "red line" on the use of chemical weapons in Syria four years ago.
In 2013, Trump had urged then-president Obama not to intervene against Assad.
Syrian Foreign Minister Walid Muallem on Thursday repeated the regime's denial it conducted a chemical strike.
"The Syrian army has not, did not and will not use this kind of weapons -- not just against our own people, but even against the terrorists that attack our civilians with their mortar rounds," he said.
Russia also stood by its longtime ally, with President Vladimir Putin warning against a rush to judgment.
Putin underlined "the unacceptability of making unfounded accusations against anyone before a thorough and impartial international investigation is carried out."
- 'Terrible responsibility' -
The UN children's agency UNICEF says at least 546 people were wounded in the suspected chemical attack.
More than 30 people were transferred across the border into Turkey for treatment, and Ankara said a preliminary probe found a link between these injuries and sarin.
If confirmed to be a chemical attack, this would be among the worst such incidents in Syria's civil war, which has killed more than 320,000 people since it began in March 2011.
Seeking to avoid a showdown between Russia and the West, the Security Council circulated a compromise resolution on a probe into the apparent chemical attack.
But the text drew a cool response from the United States and a council diplomat said the United States was showing "no flexibility."
The Organization for the Prohibition of Chemical Weapons said Thursday it has opened an "ongoing investigation" and has "initiated contact with the Syrian authorities."
Syria officially relinquished its chemical arsenal and signed the Chemical Weapons Convention in 2013 to avert military action after it was accused of an attack outside Damascus that killed hundreds.
But there have been repeated allegations of chemical weapons use since.