Trump 'turns against Assad' after chemical attack atrocity

US President Donald Trump said on Wednesday that the deadly chemical attack in Syria "crossed the line" and has changed his thinking about Syrian President Bashar al-Assad.

Trump said the chemical strike on the rebel-held town of Khan Sheikhun, which killed at least at least 86 people, was an "affront to humanity", blaming the attack squarely on the Assad regime.

"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,"

"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," he added.

Trump's statements come days after members of his administration said Assad's removal from power was no longer a US priority, drawing outrage from Assad critics.

In a statement on Tuesday, Trump blamed former President Barack Obama's "weakness" for the Khan Sheikhun attack.

Trump has yet to go into detail about what a US response to the atrocity will be - he has previously opposed deeper US military involvement in Syria's civil war.

But his statement that his attitude change 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.

The Syrian conflict began when the Baath regime, in power since 1963 and led by Assad, responded with military force to peaceful protests demanding democratic reforms during the Arab Spring wave of uprisings, triggering an armed rebellion fuelled by mass defections from the Syrian army.

According to independent monitors, hundreds of thousands of civilians have been killed in the war, mostly by the regime and its powerful allies, and millions have been displaced both inside and outside of Syria.

The brutal tactics pursued mainly by the regime, which have included the use of chemical weapons, sieges, mass executions and torture against civilians have led to war crimes investigations.