Donald Trump has raised the prospect of military action against the Syrian regime after he said that Bashar al-Assad has "crossed a lot of lines" and the US has a "responsibility" to act.
The US president said Tuesday's chemical weapons attack, which intelligence officials say was undoubtedly launched by regime war planes, was an "affront to humanity".
At least 73 people were killed, 23 of whom were children, in the sarin gas attack in the rebel-held Idlib province.
The US response to the attack is being viewed as Mr Trump's first major foreign policy test.
Speaking at a White House press conference, the President suggested that the US would now respond to the Assad regime. When asked how he planned to respond, Mr Trump told reporters: "You'll see."
Mr Trump's comments came after Nikki Haley, the US ambassador to the United Nations, condemned the UN Security Council's failure to intervene in the Syrian civil war.
She said: "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."
Mr Trump said: "A chemical attack that was so horrific in Syria, against women and small children, and even beautiful little babies, and their deaths, was an affront to humanity."
"These heinous acts by the Assad regime cannot be tolerated. The US stands with our allies across the globe to condemn this horrific attack."
The Trump administration had said only last week that US diplomatic policy on Syria was no longer focused on making Mr Assad leave power.
However, Tuesday's attack - the worst chemical attack in Syria for more than four years - has dramatically changed Mr Trump's position.
"It had a big impact on me. It was a horrible, horrible thing. I've been watching it and it doesn't get any worse than that," he said.
"My attitude to Syria and Assad has changed very much. You are now talking about a whole different level. What happened yesterday is unacceptable to me.
"You will see. They will have a message, you will see what the message will be."
One option would be for Mr Trump to order surgical strikes on regime air bases using surface to air missiles, defence sources said.
Any direct US military intervention is unlikely to involve what could develop into a protracted conflict.
Mr Trump has regularly condemned the lengthy and costly US wars in Afghanistan and Iraq in the last decade.
"I would love to have never been in the Middle East. I would love to have never seen that whole big situation start but once it started we got out the wrong way and Isis formed in that vacuum," he said on Wednesday.
The US was close to launching air strikes against Syria in 2013 following a chemical attack on the Damascus suburbs that killed up to 1,300 people.
However, President Barack Obama backed down, mainly because Parliament voted against intervention thereby depriving America of a key ally in any military action.
Mr Obama secretly authorised the CIA to begin arming Syria’s rebels fighting Mr Assad in 2013, and the US-led coalition currently provides intelligence, supplies and training to rebel groups.
While the US has directed strikes against the Islamic State of Iraq and the Levant (Isil) forces in Syria, it has not directly intervened militarily against the regime.
Mr Trump said Mr Obama's failure to act after the previous chemical attack had "set us back" and proved to be a "blank threat".
"I think the Obama administration had a great opportunity to solve this crisis a long time ago when he said the red line in the sand," he said.
"And when he didn't cross that line after making the threat I think that set us back a long ways, not only in Syria but in many other parts of the world, because it was a blank threat.
"I think it was not one of our better days as a country. I now have that responsibility and I will carry it very proudly."
In his comments Mr Trump did not once mention Russia, the Assad regime's key ally. However, Mike Pence, the US vice president, later said "all options are on the table" in how to respond to the attack and said the "time has come" for Russia to meet its obligations in eliminating chemical weapons in Syria.
Rex Tillerson, the US secretary of state, added that the Russians "really need to think carefully about their continued support of the Assad regime".
Downing Street said on Wednesday there were no plans to take military action against Mr Assad over the “appalling” chemical weapons attack on Tuesday.
Russia, Mr Assad's key remaining ally, offered an alternative explanation of the chemical attack yesterday that would shield the regime. Officials in Moscow stated 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 pin the blame on Damascus.
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 and Russia, which support opposing sides of the war, traded barbs at the UN on Wednesday.
"How many more children have to die before Russia cares?" Nikki Haley, US Ambassador, told a meeting of the Security Council before standing up and showing two photographs of victims of the attack.