Washington: US President Donald Trump warned on Monday that Moscow, Tehran and Damascus may be forced to pay a "big price" after the latest alleged chemical weapons atrocity of the Syrian civil war.
The American leader met with his cabinet and promised that "major decisions" would be made within the next 24 to 48 hours, amid mounting speculation about a US military response.
The United Nations Security Council, meanwhile, met in New York after Washington urged the international community to approve an investigation into the alleged attack on rebel-held Douma, which reportedly killed at least 40 people.
Relatedly, US Ambassador to the United Nations Nikki Haley said the United States "will respond" to a deadly chemical weapons attack in Syria regardless of whether the United Nations Security Council acts or not.
"We have reached the moment when the world must see justice done," Haley told the council.
"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," she said. "Either way, the United States will respond."
Earlier, Trump had condemned what he said had been a "heinous attack on innocent" Syrians, after reports emerged of women and children choking to death in agony, adding: "This is about humanity - it can't be allowed to happen."
Asked whether Russia's President Vladimir Putin bore some of the responsibility for the latest attack to be blamed on his ally Bashar Al Assad's regime, Trump said: "He may, and if he does it's going to be very tough.
"Everybody is going to pay a price. He will. Everybody will," he said.
White House spokeswoman Sarah Sanders later made the warning even more explicit, saying: "The president has noted that Russia and Iran also bear responsibility for these acts since they would not be possible without their material support."
On April 7, 2017, almost exactly a year ago, Trump ordered the US Navy to target a Syrian airbase with cruise missiles in the wake of a similar attack that the UN chemical weapons watchdog OPCW later pinned on Assad's regime.
But the stakes in any similar punitive strike could now be higher, with Putin warning against any "provocation and speculation on this matter."
Russian Foreign Minister Sergei Lavrov had earlier claimed that Russian specialists found no trace of a chemical attack in Douma, the largest town in the Eastern Ghouta region outside Damascus.
And Putin's spokesman Dmitry Peskov suggested that rebels could have staged the attack themselves to pin the blame on Damascus.
But Washington's main European allies stood by reports blaming Assad. France's President Emmanuel Macron has promised a "strong, joint response" and Britain has joined a growing chorus demanding action.
British Foreign Secretary Boris Johnson spoke by telephone with acting US secretary of state John Sullivan and, according to London, they agreed that "this attack bore hallmarks of previous chemical weapons attacks by the Assad regime."
With Washington and Paris expected to respond, an apparent Israeli strike on a Syrian regime air base early Monday initially raised speculation that US punitive raids had begun.
Tehran later confirmed that four of its personnel were killed in the strike, underlining the international nature of the conflict and the risk of escalation if and when the West takes action.
Rescuers and medics in Douma say more than 40 people died after an alleged poison gas attack late Saturday 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.
The Organisation for the Prohibition of Chemical Weapons (OPCW) said it was investigating the allegations but that so far only a "preliminary analysis" had been carried out.
Since February 18, Syrian regime forces have been carrying out an offensive against Ghouta that has killed more than 1,700 civilians and left Islamist rebels cornered 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 concentrated regime bombing blitz, state media on Sunday announced a deal was reached for Jaish Al Islam to leave Douma within 48 hours and release hostages.
Several buses of detainees arrived in Damascus overnight after having been freed by Jaish Al Islam, and rebels were being evacuated in a parallel operation on Monday, SANA said.
Meanwhile, an explosion killed 11 people and wounded more than 80 on Monday in the rebel-controlled Syrian city of Idlib, the Syrian Observatory for Human Rights said.
The British-based war monitoring group said the source of the blast was unclear.
The Civil Defence in Idlib, a rescue service that operates in rebel territory, gave the death toll as at least 10 civilians with 110 others injured. It caused damage in four buildings, one of them entirely.
The northwestern Idlib region remains the largest populated area of Syria in the hands of insurgents fighting the Damascus government - including both jihadist factions and nationalist FSA rebels. The dominant force there is Hayat Tahrir Al Sham, an Islamist alliance spearheaded by the former Al Qaeda affiliate in Syria.