US President Donald Trump has criticised Russia as he condemned President Assad's government for an alleged chemical weapons attack that has killed dozens of Syrian civilians.
The US president described Tuesday's attack as an "affront to humanity" that had crossed "many, many lines".
At least 86 people died in the globally condemned attack on the town of Khan Sheikhoun, including 30 children and 20 women, according to the UK-based Syrian Observatory for Human Rights.
Prime Minister Theresa May said Russia should use its influence to stop Assad bombing his own people.
Mr Trump told the New York Times the attack was a "very sad for Russia because they’re aligned" though he said he was holding the Assad regime solely responsible for a contamination it continues to deny committing.
His criticism of Russia's ally puts the US president directly at odds with Moscow's claim that Syrian rebels were to blame for the attack.
Asked about Russia's role, he said: "Well, I think it’s very disappointing. Now this was done by - the information is, this was done by Syria purely. But anybody aligned with Syria - this is very disappointing."
Mr Trump's comments came as the US Secretary of State Rex Tillerson and Vice President Mike Pence both issued warnings to Russia and the Syrian government.
Mr Tillerson said Russia needed to "think carefully about their continued support for the Assad regime" while Mr Pence said "all options are on the table" when asked on Fox News about potential moves to oust Mr Assad from power.
Mr Pence also said the time had come for Moscow to "keep the word that they made to see to the elimination of chemical weapons so that they no longer threaten the people in that country".
Mr Trump refused to confirm what response - if any - his administration would take, telling the New York Times: "I never talk about what I do militarily."
But in a separate interview, the US president hinted at action as he told reporters: "My attitude toward Syria and Assad has changed very much."
Mr Trump's comments came just a few days after Washington said it was no longer focused on making Assad leave power.
US Ambassador Nikki Haley warned on Wednesday the Trump administration was ready to take action if the UN Security Council fails to act in response to the chemical attacks.
She held up photos of victims of the attack as she accused Russia of blocking action and closing its eyes to the "barbarity" of three previous chemical attacks that investigators blamed on the Syrian government.
Mrs May described the incident as a "despicable attack" and said if it was proven it was carried out by the Assad regime it would show its "barbarism".
The prime minister said: "All those who are backing that regime, including Russia, need to use their influence to stop Assad from bombarding and dealing with his people in such a horrific way."
Foreign Secretary Boris Johnson meanwhile said all the evidence points to the Assad regime being responsible for Tuesday's attack.
The UN held emergency talks on Wednesday to discuss the fatal contamination, which was condemned as "barbaric" by world leaders.
Moscow has vowed to continue its support of the Assad regime.
A Kremlin spokesman said Russia will argue the contamination of the northwestern province of Idlib was caused by exposure to chemicals from a rebel arsenal hit by a Syrian air strike.