The United States has warned it could take unilateral action if the United Nations fails to respond to a suspected chemical attack on a rebel-held town in Syria that killed more than 80 people, including many children.

"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," US Ambassador to the UN Nikki Haley said on Wednesday.

The warning came during an emergency meeting of the UN Security Council called by France and Britain after an early morning attack on Tuesday in Khan Sheikhoun in Idlib province.

Britain, France and the US presented a draft resolution demanding a full investigation of the attack, which they blamed on the Syrian government.

But talks ended without a vote after Russia, an ally of Syrian President Bashar al-Assad, said the text was "categorically unacceptable".

Syria has denied the allegations, while Russia had blamed the rebels, saying the deaths occurred when a government shell hit a rebel chemical weapons depot.

'Horrific acts'

Haley lashed out at Moscow for failing to rein in Damascus, standing in the council chamber to hold up photographs of victims - one showing a young child lying lifeless, a mask covering his face.

"How many more children have to die before Russia cares?" she asked.

"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."

The Britain-based Syrian Observatory for Human Rights monitoring group said at least 86 people, including 30 children, were killed in the raid on Khan Sheikhoun.

Dozens more were left gasping for air, convulsing, and foaming at the mouth, doctors said.

If confirmed, it will be be the worst chemical weapons attack in Syria since 2013, when sarin gas was used on a rebel-held area of Damascus.

"If we are not prepared to act, then this council will keep meeting, month after month to express outrage at the continuing use of chemical weapons and it will not end," Haley said.

"We will see more conflict in Syria. We will see more pictures that we can never unsee."

READ MORE: 'Chemical attack' in Syria draws international outrage

The draft resolution backs a probe by the Organisation of the Prohibition of Chemical Weapons (OPCW) and demands that Syria cooperate to provide information on its military operations on the day of the assault.

Russia's Deputy Ambassador Vladimir Safronkov told the council the proposed measure was hastily prepared and unnecessary, but voiced support for an investigation.

"The main task now is to have an objective inquiry into what happened," he said.

Negotiations continued on the proposed resolutions throughout most of Wednesday. Diplomats said it could come up for a vote at the council as early as Thursday.

'Many, many lines' crossed

In a press conference at the White House later in the day, US President Donald Trump said the chemical attack had crossed "many, many lines" and had abruptly changed his thinking about Syrian President Bashar al-Assad.

Only days earlier multiple members of Trump's administration had said Assad's ouster was no longer a US priority, drawing outrage from Assad critics in the United States and abroad. But Trump said Tuesday's attack "had a big impact on me - big impact".

"My attitude towards Syria and Assad has changed very much," he said, but refused to telegraph any potential US military retaliation.

Since the attack, Trump has been under increasing pressure to explain whether it was egregious enough to force a US response.

Robert Ford, former US ambassador to Syria, expressed scepticism that Trump would resort to military action.

"As a presidential candidate he could not have been more clear that he wanted to avoid military involvement in the Syrian civil war," he told Al Jazeera. "For him to order military strikes, even limited military strikes, in response to the chemical attack in Idlib, would be a gigantic change and not one that I'm at all sure that the administration is actually going to do."

Ford said all fingers point to the Syrian government as the culprit of the attack.

"I find it laughable that governments such as Russia would suggest that rebels have a chemical weapons capacity but they always seem to use it on their own people and never on the Syrian army," he added.

Idlib hospitals overwhelmed after suspected gas attack

Trump's first reaction to the attack was to blame former president Barack Obama's "weakness" in earlier years for enabling Assad.

Obama had put Assad on notice that using chemical weapons would cross a "red line" necessitating a US response, but then failed to follow through, pulling back from planned air strikes on Assad's forces after Congress would not vote to approve them.

Trump and other critics have cited that as a key moment the US lost much global credibility.

"I now have responsibility," Trump said. "That responsibility could be made a lot easier if it was handled years ago."

Joshua Landis, director for the Centre of Middle East Studies at the University of Oklahoma, told Al Jazeera that the US would likely warn Moscow if it was to resort to using military might in Syria.

"They have to disambiguate and they have to make sure that they don’t hurt any Russian soldiers," he said.

"But there’s a wide palette of things they can do. They can bomb airports and destroy the runways so that Syrian planes can’t fly for some time, they could kill a bunch of Syrian soldiers, they could destroy command centres…we'll have to wait and see."

But Landis said it was unlikely that Trump would push for all out regime change in Syria.

"[Trump's] entire policy for the Middle East has been based on reversing the notion of regime change. He supports strong men," he said.

"Now that he's got a strong man in Assad who is violating human rights and this norm of not using chemical weapons, he has to calibrate his policy and try to insist on human rights even as he supports dictators."