Russia and Syria on Monday accused Israel of carrying out deadly missile strikes on a Syrian government airbase from Lebanon, as calls grew for international action over an alleged chemical attack on a rebel-held area.
Britain was the latest country to urge a "strong" response to accusations that at least 49 people were killed by poison gas in Douma in Eastern Ghouta near the capital.
The statement came as Damascus and Moscow blamed Israel for a missile strike early on Monday on Syria's Tayfur (T-4) airbase.
"Two F-15 planes of the Israeli army hit the airbase between 3.25am and 03.53am Moscow time with the help of eight missiles controlled remotely from Lebanese territory, without entering Syrian air space," the Russian defence ministry was quoted as saying by Russian news agencies. The Syrian state news agency also said that the missiles were fired by Israel from Lebanese airspace.
Russian Foreign Minister Sergei Lavrov said the raid was a "very dangerous development".
Fourteen fighters, including Iranians, were killed in an attack on a Syrian government airbase, a UK-based monitoring group said on Monday
"At least 14 fighters were killed in the strike on the T-4 airport, among them Iranian forces," said the Syrian Observatory for Human Rights. The group's chief, Rami Abdel Rahman, said that forces from the Assad regime backers Russia and Iran, as well as fighters from the Lebanese Hezbollah militia, are known to have presence at T-4.
The US and France warned of a strong response to "horrific chemical weapons attacks" on the rebel-held area near Damascus. Both Washington and Paris denied being behind the strike on Tayfur, which happened hours before an emergency United Nations meeting on Monday over the reported use of toxic gas — allegedly by the Syrian regime — on the town of Douma.
Israel has previously targeted Iranian units in Syria but did not comment on the latest strike.
US President Donald Trump and his French counterpart, Emmanuel Macron, spoke by phone and promised a "strong, joint response" to the chemical attack that killed dozens, the White House said on Sunday.
It said that the government of Syrian President Bashar Al Assad "must be held accountable for its continued human rights abuses".
Mr Trump had earlier taken to Twitter to issue a warning to Syrian and its allies.
"Many dead, including women and children, in mindless CHEMICAL attack in Syria," Mr Trump wrote, lashing out at Russia's Vladimir Putin, a key ally of the government.
"President Putin, Russia and Iran are responsible for backing Animal Assad. Big price to pay," he said.
Damascus and its allies have denounced the accusations, the government brushing them off as an "unconvincing broken record".
Russia's Foreign Ministry called the latest reports of a chemical attack a provocation.
"A military intervention under far-fetched and fabricated pretexts in Syria, where there are Russian soldiers at the request of the legitimate Syrian government, is absolutely unacceptable and could have the most dire consequences," it said.
Editorial: Assad is emboldened to kill by his backers
Syrian state news agency Sana first said the missile strike on the Tayfur base was a "suspected US attack," but later withdrew all reference to America.
Syrian forces, backed by Russia, have attacked Eastern Ghouta in a seven-week assault to dislodged rebels, killing more than 1,700 civilians and forcing tens of thousands to flee.
Late Saturday, Syria's White Helmets, who act as first responders in rebel-held areas of Syria, said "poisonous chlorine gas" was used on Douma — the last sliver of territory held by the opposition.
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.
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 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," Mr Lavrov said.
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 Mr Assad.