US officials dismissed Russia's assertion on Wednesday that Syrian rebels were to blame for a poison gas attack rather than president Bashar al-Assad, and signalled possible unilateral action over what Donald Trump called an "affront to humanity".
The US president said the attack, which killed at least 70 people, many of them children, "crossed a lot of lines", an allusion to his predecessor Barack Obama's threat to topple Mr Assad with air strikes if he used such arms.
It was not clear what, if any, action Mr Trump would take.
The comments, which came just a few days after Washington said it was no longer focused on making Mr Assad leave power, widened a rift between the Kremlin and Trump's White House after initial signals of warmer ties.
Western countries, including the United States, blamed Mr Assad's armed forces for the worst chemical attack in Syria for more than four years.
US intelligence officials, based on a preliminary assessment, said the deaths were most likely caused by sarin nerve gas dropped by Syrian aircraft on the town of Khan Sheikhoun on Tuesday.
A senior state department official said Washington had not yet ascertained it was sarin.
Moscow offered an alternative explanation that would shield Assad: that the poison gas belonged to rebels and had leaked from an insurgent weapons depot hit by Syrian bombs.
A senior White House official, speaking on condition of anonymity, said the Russian explanation was not credible. “We don’t believe it,” the official said.
The United States, Britain and France have proposed a draft UN Security Council resolution that would condemn the attack.
The Russian Foreign Ministry called the resolution “unacceptable” and said it was based on “fake information”.
Kremlin spokesman Dmitry Peskov said Moscow would press its case blaming the rebels for the poisoning, signalling a likely veto.
The US ambassador to the United Nations, Nikki Haley, issued what appeared to be a threat of unilateral action if Security Council members could not agree.
“When the United Nations consistently fails in its duty to act collectively, there are times in the life of states that we are compelled to take our own action,” she told the council, without elaborating.
Mr Trump described the attack as “horrible” and “unspeakable”. Asked whether he was formulating a new policy towards Syria, he told reporters: “You’ll see.”
Video uploaded to social media showed civilians sprawled on the ground, some in convulsions, others lifeless.
Rescue workers hose down the limp bodies of small children, trying to wash away chemicals. People wail and pound on the chests of victims.
The charity Medecins Sans Frontieres said one of its hospitals in Syria had treated patients "with symptoms - dilated pupils, muscle spasms, involuntary defecation - consistent with exposure to neuro-toxic agents such as sarin".
The World Health Organization also said the symptoms were consistent with exposure to a nerve agent.
Turkish president Tayyip Erdogan said the attack had killed more than 100 people. That death toll could not be independently confirmed.
"We're talking about war crimes," French UN ambassador Francois Delattre told reporters in New York.
Hasan Haj Ali, commander of the Free Idlib Army rebel group, called the Russian statement blaming the rebels a "lie" and said rebels did not have the capability to produce nerve gas.
“Everyone saw the plane while it was bombing with gas,” he told Reuters from northwestern Syria.
“Likewise, all the civilians in the area know that there are no military positions there, or places for the manufacture (of weapons).”
The incident is the first time US intelligence officials have accused Mr Assad of using sarin since 2013, when hundreds of people died in an attack on a Damascus suburb.
At that time, Washington said Mr Assad had crossed a “red line” set by then-president Obama.
Mr Obama threatened an air campaign to topple Mr Assad but called it off at the last minute when the Syrian leader agreed to give up his chemical arsenal under a deal brokered by Moscow, a decision which Mr Trump has long said proved Mr Obama’s weakness.
The new incident means Mr Trump is faced with same dilemma that faced his predecessor: whether to openly challenge Moscow and risk deep involvement in a Middle East war by seeking to punish Mr Assad for using banned weapons, or compromise and accept the Syrian leader remaining in power at the risk of looking weak.
Mr Trump has described Tuesday’s incident as “heinous actions by the Bashar al-Assad regime” and faulted Mr Obama for having failed to enforce the red line four years ago. Mr Obama’s spokesman declined to comment.
The draft UN Security Council resolution condemns the attack and presses Syria to cooperate with international investigators. Russia has the power to veto it. It has blocked seven resolutions to protect Mr Assad's government, most recently in February.
France’s foreign minister said the chemical attack showed Mr Assad was testing whether the new US administration would stand by Obama-era demands that he be removed from power.
“It’s a test. That’s why France repeats the messages, notably to the Americans, to clarify their position,” Jean-Marc Ayrault told RTL radio. “I told them that we need clarity. What’s your position?”
Mr Trump's response to a diplomatic confrontation with Moscow will be closely watched at home because of accusations by his political opponents that he is too supportive of Russian president Vladimir Putin.
US intelligence agencies say Russia intervened in the US presidential election last year through computer hacking to help Mr Trump defeat Hillary Clinton. The FBI and two congressional committees are investigating whether figures from the Trump campaign colluded with Moscow, which the White House denies.
Mr Trump’s relationship with Russia has deteriorated since the presidential election campaign, when Mr Trump praised Mr Putin as a strong leader and vowed to improve relations between the two countries, including a more coordinated effort to defeat Islamic State in Syria.
But as Russia has grown more assertive, including interfering in European politics and deploying missiles in Kaliningrad and a new ground-launched cruise missile near Volgograd in southern Russia - an apparent violation of the 1987 Intermediate Nuclear Forces (INF) Treaty - relations have cooled, US officials have said.
The chemical attack in Idlib province, one of the last major strongholds of rebels, who have fought since 2011 to topple Mr Assad, complicates diplomatic efforts to end a war that has killed hundreds of thousands of people and driven half of Syrians from their homes.
Over the past several months, Western countries, including the United States, had been quietly dropping their demands that Mr Assad leave power in any deal to end the war, accepting that the rebels no longer had the capability to topple him by force.
The use of banned chemical weapons would make it harder for the international community to sign off on any peace deal that does not remove him. Britain and France on Wednesday renewed their call for Mr Assad to leave power.