Syria's regime and Russia hit back Thursday after accusations that Damascus was behind a deadly chemical weapons attack, as pressure built for international action over what Washington has called an "affront to humanity".

Britain, France and the United States have pressed for a vote on a UN Security Council resolution to investigate dozens of civilian deaths in a northwestern Syria town, which Turkey said it suspected were the result of exposure to nerve agent sarin.

At least 86 people were killed early Tuesday in rebel-held Khan Sheikhun, and dozens more treated for convulsions, breathing problems and foaming at the mouth.

World powers have pointed the finger at Bashar al-Assad's regime, but Foreign Minister Walid Muallem repeated the regime's denial Thursday.

"The Syrian army has not, did not and will not use this kind of weapons -- not just against our own people, but even against the terrorists that attack our civilians with their mortar rounds," he said.

Russia also stood by its longtime ally, with President Vladimir Putin warning against a rush to judgement.

Putin underlined "the unacceptability of making unfounded accusations against anyone before a thorough and impartial international investigation is carried out".

More than 30 people were transferred across the border into Turkey for treatment following the incident, and Ankara said a preliminary probe found "a link between these injuries and the use of chemical weapons".

"According to the results of the initial analysis, the findings suggest the injured were exposed to a chemical substance (sarin)," its health ministry said.

After an emergency session of the UN Security Council on Wednesday, Western diplomats are expected to push for a vote as early as Thursday on a resolution demanding an investigation into the incident.

French Foreign Minister Jean-Marc Ayrault said the resolution, presented by Britain, France and the United States, remained a priority.

But "it's difficult because up to now every time we have presented a resolution, there has been a veto by Russia and sometimes by China," he added.

Geert Cappelaere, Middle East director at the UN children's agency UNICEF, said at least 27 children were killed and 546 wounded in the suspected attack.

If confirmed as an attack, it would be among the worst incidents of chemical weapons use in Syria's civil war, which has killed more than 320,000 people since it began in March 2011.

It has also prompted an about-face from US President Donald Trump, who in 2013 urged then-president Barack Obama not to intervene against Assad after a major suspected chemical attack.

Senior US officials had also recently suggested it was no longer a priority that Assad be removed from power.

Trump described the alleged attack as an "affront to humanity" and warned it had changed his view of Assad.

"It crossed a lot of lines for me," he said, alluding to Obama's failure to enforce his own 2013 "red line" on the use of chemical weapons in Syria.

As she held up pictures of lifeless children at the UN on Wednesday, US ambassador Nikki Haley warned of unilateral action if the UN failed "in its duty to act collectively".

The draft UN resolution backs a probe by the Organization for the Prohibition of Chemical Weapons and demands Syria provide information on its operations.

The OPCW said Thursday it has opened an "ongoing investigation" into the suspected chemical attack, and has "initiated contact with the Syrian authorities" as part of the probe.

Earlier, Muallem said such an investigation "must guarantee that it is not politicised, that it has broad geographic representation and that it is launched from Damascus, not Turkey".

Britain, France and the United States asked the Security Council to hold a vote later Thursday on the resolution, diplomats said.

But it remained unclear whether Russia would support the measure, which was slightly revised after negotiations in the past two days.

Russia has previously used its veto seven times to shield Syria.

France warned Russia it would face a "terrible responsibility in front of history" if it vetoed it.

Turkey said Russia's support of the Syrian regime was "utterly wrong", in Ankara's most bitter recent attack on Moscow after a dramatic warming of ties in recent months.

Syria officially relinquished its chemical arsenal and signed the Chemical Weapons Convention in 2013 to avert military action after it was accused of an attack outside Damascus that killed hundreds.

But there have been repeated allegations of chemical weapons use since.

Analysts said it was unclear whether the Trump administration would follow through with its threats of action.

"We have no precedent to use to assess whether the Trump administration's words yesterday were bluster or a representation of genuine threat," said Charles Lister of the Middle East Institute.

"Trump... was exposed to the horrific footage that we all saw and quite clearly that had a transformative effect on him

"Now we need to wait to see whether that transforms into real policy shifts or not."