US and Russia in row over Syria before diplomatic talks in Moscow
Russia has been told it must side with the US and like-minded countries or embrace Syrian President Bashar Assad, Iran and militant group Hezbollah.
The ultimatum came from US secretary of state Rex Tillerson as he arrived in Moscow following urgent meetings in Italy with top diplomats.
It was not clear what the punishment would be for a Russian government that has used its military might to help Assad and his array of allies score a series of battlefield successes in their six-year war with Syrian opposition groups.
Russian President Vladimir Putin immediately showed that he would not back down, saying Russia knew about planned "provocations" to blame Syria's government for using chemical weapons.
He said the UN should first investigate the attack.
"It reminds me of the events in 2003 when US envoys to the Security Council were demonstrating what they said were chemical weapons found in Iraq," Mr Putin told reporters. "We have seen it all already."
Mr Tillerson is in Moscow to meet with Russian officials about the Syria civil war.
It is the first official trip to Russia by a member of President Donald Trump's cabinet.
Before heading to Russia, Mr Tillerson told reporters that Moscow had either failed to take seriously its obligation to rid Syria of chemical weapons or had been incompetent, but, he said, the distinction "doesn't much matter to the dead".
His trip follows Monday's claim by a senior US official that Washington has concluded Russia knew in advance of Syria's chemical weapons attack.
The official did not offer concrete proof and others in the Trump administration cautioned that no final determination of Russia's foreknowledge had been made.
"We cannot let this happen again," Mr Tillerson said of the chemical attack.
"We want to relieve the suffering of the Syrian people. Russia can be a part of that future and play an important role," Mr Tillerson added in remarks to reporters. "Or Russia can maintain its alliance" with Syria and Iran.
Russia's foreign ministry said in a statement that it hoped for "productive talks."
It said the outcome of the discussions is important not only for the US-Russian relationship, but "for the overall atmosphere on the world stage".
The trade of accusations followed a long-set pattern for Syria's civil war.
Under president Barack Obama, the United States accused Russia of supporting Assad in war crimes against civilians and of employing a sophisticated disinformation campaign to protect its ally.
Assad and Russia have accused the US of supporting terrorist groups, who have staged atrocities to gain international sympathy for their cause.
There was one key difference: After the recent launch of 59 Tomahawk cruise missiles on a Syrian air base, President Donald Trump can now offer each a credible threat that the US will use force in Syria if the stalemate persists.
The Trump administration, however, has offered mixed messages about whether Assad must surrender power - and when.
Mr Tillerson said it was clear the US saw no role for Assad in Syria's future, given that he had lost legitimacy.
"It is clear to all of us that the reign of the Assad family is coming to an end," he said. "But the question of how that ends and the transition itself could be very important in our view to the durability, the stability inside of a unified Syria."
Such clarity on Assad's future is not shared by Russia, which insists that Syria's leadership cannot be a matter for outside powers.
Mr Tillerson said Russian-sponsored ceasefire talks could generate momentum towards broader talks about a political transition - if they create a durable ceasefire. The idea is for political talks to follow, but Mr Tillerson acknowledged the lack of progress so far.
Mr Tillerson spoke after a meeting of the "like-minded" countries was hastily arranged on the sidelines of the summit of the G7 industrialised economies in Italy.
A key focus since the chemical attack has been on increasing pressure on Russia, Assad's strongest ally, which has used its own military to keep Assad in power.
The US and others have said that Russia bears responsibility for the deaths of civilians at the hands of Assad given Moscow's role in guaranteeing the 2013 deal in which Assad was supposed to have given up his chemical weapons arsenal.
That accusation will hang over Mr Tillerson's visit to Moscow, where he plans to meet Russia's foreign minister, Sergey Lavrov.
The United States has sought to minimise expectations for the trip or the likelihood that the US will leave with any Russian concessions on Syria.
Instead, the US is hoping to use the visit - the first by a Trump cabinet official to Russia - to convey expectations to Moscow and allow Russians time to respond.
Though intended to punish Assad for a chemical weapons attack, the US strikes last week also served to refocus the world's attention on the bloody war in Syria.
In Italy, diplomats spoke of possible new sanctions on Syria's and Russia's militaries, and additional US military action if Assad's forces continue attacking civilians.