US Ambassador Nikki Haley has warned that the Trump administration will take action against chemical attacks in Syria that bear "all the hallmarks" of President Bashar Assad's government if the UN Security Council fails to act.

Haley urged the council at an emergency meeting to immediately approve a resolution drafted by the US, Britain and France that condemns and threatens consequences for the use of chemical weapons, especially in Tuesday's attack that killed dozens of people in rebel-held Idlib province.

"There are times at the United Nations when we are compelled to take collective action," she said.

"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."

"For the sake of the victims, I hope the rest of the council is finally willing to do the same," she added.

Haley spoke on Wednesday after Russian Foreign Ministry spokeswoman Maria Zakharova announced Moscow's opposition to the draft resolution.

She called it "categorically unacceptable" because "it runs ahead of the investigation results and names the culprit, Damascus".

"The main task now is to have an objective inquiry into what happened," Russia's deputy UN ambassador Vladimir Safronkov told the Security Council.

"Up to now all falsified reports about this incident have come from the White Helmets or the Syrian Observatory for Human Rights based in London which have been discredited long ago."

Britain's UN Ambassador Matthew Rycroft also told the council that Tuesday's attack "bears all the hallmarks" of President Bashar Assad's regime.

"We have every indication that this was a sustained attack using aircraft over a number of hours," Rycroft said.

"We see all the signs of an attack using a nerve agent capable of killing over a hundred people and harming hundreds more."

Holding up photos of victims of the attack, Haley accused Russia of blocking action and closing its eyes to the "barbarity" of three previous chemical attacks that investigators blamed on the Syrian government by vetoing a resolution in late February that would have imposed sanctions on those responsible.

"The truth is that Assad, Russia, and Iran have no interest in peace," she said. "The illegitimate Syrian government, led by a man with no conscience, has committed untold atrocities against his people for six years."

Haley said Assad has shown he isn't interested in participating in "a meaningful political process, Iran has reinforced Assad's military, and Russia has shielded Assad from UN sanctions."

The resolution drafted by the US, Britain and France would condemn the use of chemical weapons in Syria, especially on Tuesday, "in the strongest terms" and back an investigation by the international chemical weapons watchdog, the Organisation for the Prohibition of Chemical Weapons.

It reminds the government that it is also obligated to immediately provide investigators with access to air bases where they believe chemical weapons attacks may have been launched.