Damascus - Calls grew on Monday for an international response to an alleged chemical weapons attack on a rebel-held Syrian town, with the United States warning that it was not excluding military action.
US Defence Secretary Jim Mattis said on Monday that Washington does not "rule out anything" in terms of a response, despite Russia arguing against jumping to any "dangerous" conclusions over the suspected attack.
With tensions high, the spectre of Western military action was raised by deadly missile strikes on a regime airbase, but Damascus and Moscow said Israel was responsible.
The crisis was due to be discussed later on Monday at an urgent UN Security Council meeting, with the US and France promising a "strong, joint response" to the suspected gas attack on Saturday in Douma, a battered opposition-held town near Damascus.
President Donald Trump - who last year launched a missile strike on a regime base after another alleged chemical attack - warned after the latest accusations that there would be a "big price to pay".
Damascus and Moscow have denied any use of chemical weapons, but French President Emmanuel Macron's office said he and Trump had shared information "confirming" the gas attack.
Britain was the latest country to urge action on Monday, with Foreign Secretary Boris Johnson calling for a "strong and robust international response".
Russia, which has repeatedly said that rebels in Syria could be a preparing a "provocation" to prompt foreign military action, warned against jumping to conclusions.
"It's necessary to examine very carefully what happened in Douma. And it goes without saying that without this information, making any deductions is wrong and dangerous," Kremlin spokesman Dmitry Peskov told journalists.
Rescuers and medics in Douma say at least 48 people died after a "poisonous chlorine gas attack" late on Saturday in Douma, the last rebel-held pocket of the onetime opposition stronghold of Eastern Ghouta.
Access to the area, which has faced weeks of regime bombardment, is limited and there has been no way to independently verify the accounts.
The Organisation for the Prohibition of Chemical Weapons, a UN watchdog, said it was investigating the allegations but that so far only a "preliminary analysis" had been carried out.
Russian Foreign Minister Sergei Lavrov said Russian experts had visited the site of the alleged attack and "did not find any trace of chlorine or any other chemical substance used against civilians".
Both Damascus and Moscow had warned against using the allegations to justify military action against President Bashar al-Assad.
It briefly appeared in the early hours of Monday that action had been taken, as Syrian state media reported missile strikes on an airbase, but blame later fell on Israel which has carried out repeated strikes on regime targets.
Initially pointing the finger at Washington, state news agency SANA said the missiles had hit the T-4 airbase in central Syria.
Washington and Paris denied having carried out the strikes and SANA and the Russian army said later that Israeli F-15s had fired several missiles at the base from Lebanese airspace.
The Syrian Observatory for Human Rights, which monitors the country's conflict, said 14 fighters were killed, including Syrian army officers and Iranian forces.
Israel declined to comment on the latest strike.
Nick Heras, an analyst at the Center for a New American Security, said Israel appeared to have seized on a moment of international outrage over Douma to strike Syria.
"On the eve of potential US and French strikes against Assad in response to the Douma gas attack, it is likely the Israelis already had the intention to strike at the T-4 base, and timed their attack to maximise the destruction they could inflict on Iranian forces," he told AFP.
Israel has repeatedly warned it will not accept its arch-foe Iran entrenching itself militarily in Syria.
In February, it accused Iranian forces at the T-4 base of sending a drone into Israeli territory. After targeting Iranian units in Syria in retaliation, an Israel F-16 was shot down by Syrian anti-aircraft fire in one of the conflict's most notable escalations.
Israel then carried out what it called "large-scale" raids on Syrian air defence systems and Iranian targets, which reportedly included T-4.
Lebanon's National News Agency on Monday said Israeli warplanes were flying near the country's border with Syria.
Backed by Moscow, Assad has waged an assault since February 18 on Ghouta that has killed more than 1,700 civilians and left Islamist rebels cornered in their last holdout of Douma, Ghouta's largest town.
After capturing most of Ghouta, Syria and its ally 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 it was holding.
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.