US President Donald Trump warned Wednesday that Bashar al-Assad's Syrian regime had crossed a line with its latest alleged chemical attack and faces a US response.
Trump described the strike on the rebel-held town of Khan Sheikhun as an "affront to humanity" and warned it had changed his view of the Russian-backed Syrian leader.
Previously the White House has said its sole focus in Syria is defeating the jihadist Islamic State group, not on ending Assad's civil war against opposition fighters.
But Trump and other senior US officials said that the latest attack, which doctors say caused the agonizing deaths of at least 72 people, had changed the calculus.
And he renewed his criticism of his predecessor Barack Obama who in 2013 famously failed to take action after Assad crossed a "red line" with a previous chemical attack.
"It crossed a lot of lines for me," Trump said, at a joint White House news conference with Jordan's King Abdullah.
"When you kill innocent children, innocent babies, little babies ... that crosses many, many lines, beyond a red line, many, many lines," he warned.
"I will tell you, it's already happened, that my attitude towards Syria and Assad has changed very much ... You're now talking about a whole different level."
Trump did not go into detail about what a US response to the atrocity will be -- and he has previously opposed deeper US military involvement in Syria's civil war.
But his statement that his attitude had changed came after the US ambassador to the United Nations, Nikki Haley, had warned of unilateral American action.
"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 said.
- Lifeless victims -
The warning came during an emergency meeting of the UN Security Council called by France and Britain after the attack was carried out in the early hours on Tuesday.
Haley lashed out at Russia for failing to rein in its ally Syria, standing in the UN Security Council with photographs of lifeless victims, including children.
"How many more children have to die before Russia cares?" she demanded.
"If Russia has the influence in Syria that it claims to have, we need to see them use it," she said. "We need to see them put an end to these horrific acts."
At least 72 people, among them 20 children, were killed in Khan Sheikhun, and dozens more were left gasping for air, convulsing, and foaming at the mouth, doctors said.
It is thought to be the worst chemical weapons attack in Syria since 2013, when sarin was used.
US officials have not said what chemical agents were used, but Trump said it was "a chemical gas that is so lethal, people were shocked to hear what gas it was."
The attack has strained the already tense relations between the US and Russia, just days before Secretary of State Rex Tillerson is due to visit Moscow next week.
At the United Nations, Britain, France and the United States have presented a draft resolution demanding a full investigation of the attack.
Russia, Syria's main diplomatic and military partner along with Iran, said the text was "categorically unacceptable."
The draft backs a probe by the Organization of the Prohibition of Chemical Weapons (OPCW) and demands Syria provide information on its operations.
Russia's Deputy Ambassador Vladimir Safronkov told the UN council that the proposed resolution was hastily prepared and unnecessary, but voiced support for an inquiry.
"The main task now is to have an objective inquiry into what happened," he said.
Negotiations were continuing on the draft text.