US President Donald Trump was poised Tuesday to decide on possible military action against the Syrian regime, after vowing to respond "forcefully" to the latest alleged chemical atrocity in the country's bloody civil war despite strong warnings from Damascus-ally Russia.

The United States, backed by Britain and France, has said it is ready to act with or without support from the United Nations, where the Security Council was to vote as early as Tuesday on rival US and Russian proposals to probe chemical attacks in Syria.

Trump met his cabinet and then dined with top generals Monday, telling reporters "we have a lot of options militarily and we'll be letting you know pretty soon... probably after the fact."

Washington and Moscow set out starkly different positions at a stormy emergency session at the UN Security Council, where Russia's ambassador Vassily Nebenzia warned that the possibility of US military action was "very, very dangerous."

Nebenzia rejected a US call for the world body to set up a probe to identify the perpetrators of what rescuers called a "poisonous chlorine gas attack" late Saturday in the enclave of Douma, near Damascus.

But US Ambassador Nikki Haley said the "world must see justice done."

"History will record this as the moment when the Security Council either discharged its duty or demonstrated its utter and complete failure to protect the people of Syria," Haley declared.

Diplomats said the United States was pushing for a vote on Tuesday on its draft resolution setting up a Syria gas attacks inquiry, but Russia could veto the measure.

Trump slammed the "horrible attack" and vowed to respond "forcefully."

"We're going to make a decision tonight or very shortly thereafter," he told reporters.

Rescuers and medics in Douma say more than 40 people died after the suspected poison gas attack in the last rebel-held pocket of the one-time opposition stronghold of Eastern Ghouta.

Access to the area, which has faced weeks of regime bombardment, is limited and AFP has not independently verified the accounts.

In April last year, Trump launched a cruise missile strike against a Syrian air base after a previous chemical weapons attack UN monitors later pinned on Bashar al-Assad's regime.

'Very dangerous'

The punitive raid did not draw US forces any deeper into Syria's civil war, nor did it dissuade Syria or its Russian and Iranian backers from pursuing their campaign against rebel groups.

And any new action raises the risk of open conflict with Russia which, like Iran, has deployed forces to defend Assad's regime.

Nebenzia, reflecting earlier warnings from President Vladimir Putin's Kremlin, said: "From what we hear now, I am afraid they are looking for a military option, which is very, very dangerous."

Earlier, Trump was asked whether Putin bore some of the responsibility for the latest attack. "Everybody is going to pay a price. He will. Everybody will," the American president warned.

Washington's main European allies stood by reports blaming Assad.

Trump spoke to France's President Emmanuel Macron for the second time in two days over the alleged attacks, with the pair expressing a "desire for a firm response from the international community to these new violations of the chemical weapons ban," according to the Elysee Palace.

British Foreign Secretary Boris Johnson spoke with the acting US secretary of state and, according to London, they agreed that "this attack bore hallmarks of previous chemical weapons attacks by the Assad regime."

As the UN debate took place, a huge blast tore through a multi-story building in the jihadist-held city of Idlib, in northern Syria.

The cause of the explosion was not immediately apparent, but the Britain-based Syrian Observatory for Human Rights said 13 people had been killed and another 80 wounded.

Investigation launched

The Organisation for the Prohibition of Chemical Weapons said it was investigating the Douma attack allegations, but that so far only a "preliminary analysis" had taken place.

Syrian regime forces have carried out an offensive against Ghouta since February 18 that has killed more than 1,700 civilians and cornered rebels in their last holdout of Douma.

After capturing most of Ghouta, Syria and Russia secured two negotiated withdrawals last month that saw 46,000 rebels and civilians evacuate.

Following fraught negotiations and a regime bombing blitz, state media announced Sunday a deal for Jaish al-Islam to leave Douma within 48 hours and release hostages.

A group of detainees arrived in Damascus overnight Monday to Tuesday after being freed by Jaish al-Islam, state news agency SANA said.

In a parallel operation, 65 buses carrying fighters and civilians left the outskirts of Ghouta and headed to northern Syria, according to the Syrian Observatory for Human Rights.