The US has warned Syrian President Bashar al-Assad it is prepared to take further military action if he uses chemical weapons again amid mounting tensions with Russia over air strikes.
President Donald Trump unleashed a barrage of cruise missiles on a Syrian air base in retaliation for the regime's "barbaric" use of chemical weapons against its own people.
Nikki Haley, the US ambassador to the United Nations, told an emergency meeting of the world body's Security Council that Washington had taken "a very measured step" but was ready to go further.
She said: "We are prepared to do more. But we hope that will not be necessary.
"It is time for all civilised nations to stop the horrors that are taking place in Syria and demand a political solution."
In August 2012, US President Barack Obama said the use of chemical weapons would be a "red line" that would change his stance on intervening in the civil war, but failed to take military action against the regime.
US vice president Mike Pence insisted "we drew the line", according to Sebastian Gorka, deputy assistant to the president of the United States.
Mr Gorka told BBC Radio 4's Today programme: "There was a national security imperative and there was a moral one but, let's not forget, there was a messaging imperative.
"I just bumped into the vice president and he summarised it explicitly for me. He said 'we drew the line'. You do not get away as a government using weapons of mass destruction against your own citizens, women and children.
"That is a message for every nation of the world that is thinking about weapons of mass destruction acquisition or use. The message could not be clearer."
Russian prime minister Dmitry Medvedev said the strike had brought Moscow and Washington to the verge of a "military clash", adding that the action was "good news for terrorists".
He wrote on Facebook that Mr Trump, who campaigned for the presidency on a platform of non-interventionism, had quickly broken his promises now he was elected.
US-Russia relations are now "completely ruined" after the attack on what he called the "legitimate" Syrian government, he added.
But Mr Gorka denied good relations with Moscow were over.
"No, not at all," he said. "The fact is terrorism of the jihadi variety remains a threat to both our countries. Russia remains a geo-strategically important nation."
Russia and Britain clashed at the tense round of UN talks in New York.
The UK's ambassador said President Assad had been "put on notice" and claimed Moscow had been left humiliated by its failure to bring him to heel.
But Russia accused Britain of "colonial hypocrisy" and "lies" as it warned against military involvement in Syria.
Russia's UN representative Vladimir Safronkov said: "Stop putting forward these unprofessional arguments and accusations against my country. These are not diplomatic. These are lies.
"Don't even try to get into fights in the Arab world. Nothing will work and nothing will be achieved.
"All Arab countries recall your colonial hypocrisy."
Russia has consistently denied that Syrian forces used chemical weapons, claiming the incident at Khan Sheikhoun was caused by a hit on a rebel chemical weapons plant, a claim dismissed by the West.
US Secretary of State Rex Tillerson said of Russia's response: "It indicates their continued support for the Assad regime and, in particular, their continued support for a regime that carries out these type of horrendous attacks on their own people.
"So I find it very disappointing, but, sadly, I have to tell you, not all that surprising."
Russia accused the US of violating international law but Mr Trump said the dramatic strike from US warships in the Mediterranean was in the "vital national security interest".
Russian foreign minister Sergei Lavrov said the Americans had used a "far-fetched pretext" to justify the attack while the defence ministry in Moscow said it would be helping its Syrian ally strengthen its air defences.
The Federation's defence ministry spokesman Major General Igor Konashenkov played down the impact of the strike on the base at Shayrat, claiming fewer than half the missiles reached the air base, destroying just six Syrian MiG-23 fighters and leaving the runway intact.
The surprise barrage of 59 cruise missiles in the early hours of Friday, UK time, was the first time the US has struck directly against the Syrian government.
In an emotive broadcast, Mr Trump said he was responding to the regime's attack - believed to have involved sarin nerve agent - on the rebel-held town of Khan Sheikhoun which left at least 72 people dead, including 20 children.
Downing Street was swift to offer its backing for the US action, which was also supported by Israel, Australia, France, Germany and European Council president Donald Tusk.