The US has accused Syrian President Bashar al-Assad's Government of going "beyond a red line" with a poison gas attack on civilians, saying "all options" are now on the table.

Key points: US intelligence believe deaths caused by sarin nerve gas

US intelligence believe deaths caused by sarin nerve gas Russia blames rebels for attack

Russia blames rebels for attack US, Britain, France propose UN resolution blaming Damascus

US, Britain, France propose UN resolution blaming Damascus Trump says his "attitude toward Assad has changed very much"

More than 70 people, including dozens of women and children, died in what medical agencies say was probably a sarin gas attack on the town of Khan Sheikhoun.

"I will tell you, what happened yesterday is unacceptable to me," Mr Trump told reporters.

"And I will tell you, it's already happened that my attitude toward Syria and Assad has changed very much."

Mr Trump declined to say how or whether his administration would respond but said the attack "crosses many, many lines," an allusion to his predecessor Barack Obama's threat to topple Mr Assad if he crossed a "red line" by using chemical weapons.

Sorry, this video has expired Donald Trump speaks about the Syrian gas attack

"You're now talking about a whole different level," he said, adding that "it is now my responsibility to [address the Syrian conflict]".

In an interview with the New York Times ahead of the press conference, Mr Trump was asked what the chemical attack meant for Russia's role in the Syrian conflict as a supporter of Mr Assad.

"I think it's a very sad day for Russia because they're aligned, and in this case, all information points to Syria that they did this," he said.

Shortly after, US Vice-President Mike Pence and Secretary of State Rex Tillerson doubled down on Mr Trump's comments.

In an interview with Fox News, Mr Pence said that "all options are on the table" for a US response to the attack, while Mr Tillerson said that there was "no doubt" Mr Assad was responsible for the strike.

"We think it's time that the Russians really need to think carefully about their continued support of the Assad regime," Mr Tillerson said.

US officials rejected Russia's assertion that Syrian rebels were to blame for the attack.

Russia says Obama's 'red line' provoked rebels

Sorry, this video has expired US ambassador Nikki Haley addresses the UN

Earlier in the day, Western countries angrily condemned the Syrian Government during an emergency UN Security Council meeting, blaming it for the attack.

"Assad, Russia and Iran have no interest in peace. The illegitimate Syrian Government, led by a man with no conscience, has committed untold atrocities against his people," US ambassador to the UN Nikki Haley told the 15-member council.

Ms Haley issued what appeared to be a threat of unilateral action if Security Council members could not agree.

"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," Ms Haley said.

She did not signal what sort of action could be taken.

Syria's Government has denied responsibility for the attack.

Sorry, this video has expired GRAPHIC CONTENT WARNING: Suspected Syrian Government chemical attack kills more than 50.

In February, Syria's ally Russia, backed by China, cast its seventh veto to protect Mr Assad's Government from council action, blocking a bid by Western powers to impose sanctions over accusations of chemical weapons attacks.

Deputy Russian UN ambassador Vladimir Safronkov told the council that former US president Barack Obama's 2012 threat of military action if a "red line" was crossed and chemical weapons were used in Syria had provoked such attacks.

"That decision served as a starting point for future provocations by terrorists and extremist structures with the use of chemical weapons, they sought to discredit the official Damascus regime and to create a pretext for the use of military force against a sovereign state," Mr Safronkov said.

But Australia's Foreign Minister Julie Bishop said Syria needed a transition away from the regime of Mr Assad as part of any solution to the conflict wracking the country.

Ms Bishop said international condemnation must aim to change Russia's backing of the Syrian regime.

"If Russia has the influence it maintains it does, then it must step up and take responsibility for the actions of the Assad regime," she said.

Sorry, this video has expired Russia's deputy ambassador Vladimir Safronkov speaks at the UN

Medics say attack used sarin gas, more victims found in cave

US intelligence officials, based on a preliminary assessment, think the Khan Sheikhoun deaths were most likely caused by sarin nerve gas dropped by Syrian aircraft.

International health workers in Syria have been examining the victims of the attack and said they exhibited symptoms "consistent with exposure to neuro-toxic agents".

The charity Medicins Sans Frontieres said one of its hospitals had treated patients with symptoms including dilated pupils, muscle spasms and involuntary defecation.

MSF said this was consistent with exposure to neuro-toxic agents such as sarin.

The World Health Organisation also said it believed the symptoms of victims were consistent with exposure to a nerve agent.

Rescue workers in Khan Sheikhoun reported finding three more victims, saying the bodies of a woman and two children were found on Wednesday in a cave where they had been taking shelter from the poison gas.

Moscow offered an alternative narrative for the deadly incident: 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.

'What is your plan?'

Sorry, this video has expired People return to Khan Sheikhoun after the gas attack

Speaking in the council, British UN ambassador Matthew Rycroft asked Russia: "What is your plan? What is your plan to stop these horrific senseless attacks? We had a plan and we had the support and you rejected it to protect Assad."

Mr Safronkov responded by saying Russia had more than one plan, but the first one was to fight terrorism.

Mr Rycroft said the Russian vetoes sent Mr Assad a message of encouragement and Tuesday's attack was "the consequence".

Aya Fadl lies on a bed with an oxygen mask to heal breathing difficulties following the attack in Khan Sheikhoun. ( AP: Aya Fadl )

An investigation by the UN and the Organisation for the Prohibition of Chemical Weapons (OPCW) found Syrian government forces were responsible for three chlorine gas attacks in 2014 and 2015 and that Islamic State militants had used mustard gas.

The United States, Britain and France have proposed a draft UN Security Council resolution that would pin the blame on Damascus, and push Syria to provide an international inquiry with flight plans and logs for Tuesday, the names of all helicopter squadron commanders and access to air bases.

Russia has described the draft resolution as "unacceptable" and based on "fake information".

Mr Assad had agreed in 2013 to give up his chemical arsenal under a deal brokered by Russia and the United States.

ABC/wires