Syria and Russia accused Israel on Monday of carrying out a deadly bombing raid on a Syrian military airport, as calls grew for international action over an alleged chemical attack on a rebel held-town.

Britain was the latest country to urge a “strong” response to accusations that dozens of people were killed by poison gas in Douma, a battered opposition-held town near the capital.

The escalating pressure came as Damascus and Moscow blamed Israel for an early morning missile strike on Syria’s T-4 airbase.

Syrian state news agency SANA said Israeli F-15 aircraft had fired several missiles at the base from Lebanese territory.

Russia’s army said a pair of Israeli F-15s had fired eight missiles at the base. Five were destroyed by air defence systems but three hit a western part of the facility, it said.

Russian Foreign Minister Sergei Lavrov called the raid a “very dangerous development”.

The Syrian Observatory for Human Rights, which monitors the country’s conflict, said 14 fighters were killed, including Syrian army officers and Iranian forces.

Forces from regime backers Russia and Iran, as well as fighters from the Lebanese Hezbollah militia, are known to have a presence at T-4, said Observatory chief Rami Abdel Rahman.

Both Washington and Paris denied carrying out Monday’s raid.

Israel has previously targeted Iranian units in Syria, but declined to comment on the latest strike.

US forces a year ago fired a volley of cruise missiles at the government’s Shayrat air base in retaliation for another suspected chemical attack in April 2017.

‘Robust response’

Syria has been accused multiple times of using toxic weapons including sarin gas in the country’s seven-year war, which has killed more than 350,000 people.

Pressure was mounting over the latest accusations that it killed dozens of people on Saturday with a toxic gas attack on Douma, the last rebel-held town in the Eastern Ghouta suburb of the capital.

Rescuers and medics said at least 48 people died after showing symptoms consistent with exposure to “poisonous chlorine gas”, including foaming at the mouth and difficulty breathing.

British Foreign Secretary Boris Johnson on Monday called for a “strong and robust international response” to the attack, after similar calls by Paris and Washington.

US President Donald Trump had warned there would be a “big price to pay” for the attack, and had vowed with French counterpart Emmanuel Macron to react strongly.

The French presidency said the two leaders shared information “confirming” the use of chemical weapons and would coordinate their efforts at a UN Security Council meeting set for 11:30am (1530 GMT) on Monday.

The UN’s chemical weapons watchdog said it had “made a preliminary analysis of the reports of the alleged use of chemical weapons immediately after they were issued”.

Ahmet Uzumcu, who heads the Organisation for the Prohibition of Chemical Weapons, said more evidence was being gathered “to establish whether chemical weapons were used”.

The OPCW’s fact-finding mission for Syria was established in April 2014 to confirm chemical weapons use, but it does not have the mandate to establish who is responsible.

Russia said on Monday it had carried out its own probe and concluded there was no evidence.

“Our military specialists have visited this place . . . and they did not find any trace of chlorine or any other chemical substance used against civilians,” Lavrov said.

‘Maximise’ cost to Iran?

Syria and Russia have fiercely denounced the allegations of chemical use in Douma as “fabrications,” and had warned against using them to justify military action against President Bashar al-Assad.

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 said.

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 a seven-week assault 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 with a military assault, 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.