Russia and Syrian officials blamed Israel for carrying out an attack on a Syrian air base near Homs on Monday. Israel, which has struck Syrian army locations many times in the course of its neighbor's seven-year-old civil war, has not confirmed nor denied mounting the raid.
The attacked followed reports of a poison gas attack by President Bashar al-Assad's forces on a rebel-held town.
Israeli officials said the Tiyas, or T-4, air base was being used by troops from Iran and that Israel would not accept such a presence in Syria by its arch foe.
The Syrian Observatory for Human Rights, a British-based monitor, said at least 14 people were killed including some fighters of various nationalities, a reference to Iranian-backed Shiite militia members, mostly from Iraq, Lebanon and Iran fighting alongside the Syrian army.
The attack demonstrated the multi-faceted nature of the Syria conflict, which started in March 2011 as an uprising against Assad and now involves several countries and various insurgent groups in a web of alliances.
It took place hours after US President Donald Trump warned of a "big price to pay" following the reports of a poison gas attack on the rebel-held town of Douma which killed dozens of people, including children.
Trump referred in a Tweet to "Animal Assad" and criticized Russia and Iran for backing the Syrian leader, directly naming Russian President Vladimir Putin.
Many dead, including women and children, in mindless CHEMICAL attack in Syria. Area of atrocity is in lockdown and encircled by Syrian Army, making it completely inaccessible to outside world. President Putin, Russia and Iran are responsible for backing Animal Assad. Big price...— Donald J. Trump (@realDonaldTrump) April 8, 2018
Damascus denied its forces had launched any chemical assault and Russian Foreign Minister Sergei Lavrov said such allegations were false and a provocation. Lavrov also said the strike on the T-4 base was a dangerous development.
Syrian state TV initially said the United States was suspected of carrying out the attack on T-4. Washington denied this, and France, which in February had said it would strike in the event of a chemical weapon attack on civilians by Syrian government forces, also said its forces were not involved.
The Russian military, whose support for Assad has turned the war in his favor, said two Israeli F-15 war planes carried out the strike. Interfax news agency cited the Russian Defense Ministry as saying Syrian air defense systems had shot down five of eight missiles fired.
Syrian state media, citing a military source, carried a similar report. "The Israeli aggression on the T4 airport was carried out with F-15 planes that fired several missiles from above Lebanese land," state news agency SANA said.
The Israeli government had no immediate comment.
Israel has accused Damascus of allowing Iran to set up a complex at the T-4 base to supply arms to its ally, Lebanon's Hezbollah, and in the past its forces have hit convoys and bases of Iranian-backed militias that fight alongside Assad's troops.
Defense analysts say a number of Russian troops are also based there and jets fly regular sorties from T-4.
Israeli opposition lawmaker Yair Lapid told Army Radio: "Israel will not accept an Iranian military presence in Syria and will not accept Iran's creeping entrenchment in Syria and this has costs."
Housing Minister Yoav Galant, while also not confirming that Israel had carried out the attack, said: "We have clear interests in Syria. We laid down red lines there, which said that we would not allow Syrian land to be a springboard for game-changing weaponry to Lebanon, we would not allow the building of an Iranian army in Syria and we would not allow the opening of another front on the Golan Heights.
"In this context we are taking action with all means, over time."
As international officials worked to try to confirm Saturday's chemical attack on Duma, a Syria medical relief group said at least 60 people had been killed there and more than 1,000 injured in several sites.
The toll is likely to rise, said the Union of Medical Care Organizations (UOSSM), a group of international aid agencies.
"The numbers keep rising as relief workers struggle to gain access to the subterranean areas where gas has entered and hundreds of families had sought refuge," the group said.
Foam at the mouth
The Syrian American Medical Society and the civil defense service, which operates in rebel-held areas, said 49 people had been killed in the suspected gas attack.
The Syrian opposition blamed the suspected chemical attack on government forces, who launched an air and ground assault on Duma, the last rebel-held town in the eastern Ghouta district, on Friday.
One video shared by activists showed bodies of about a dozen children, women and men, some with foam at the mouth.
Reuters could not independently verify the reports.
The United States launched a missile strike on a Syrian air base a year ago in response to the killing of dozens of civilians in a sarin gas attack in an opposition-held town in northwest Syria. The gas attack was blamed on Assad.
US government sources said Washington's assessment of Saturday's attack was that chemical weapons were used. The European Union also said evidence pointed to the use of chemical weapons by Assad's forces.
The UN Security Council will meet twice on Monday following rival requests by Russia and the United States.
UN war crimes investigators had previously documented 33 chemical attacks in Syria, attributing 27 to the Assad government, which has repeatedly denied using the weapons.
UOSSM said doctors could not determine the origin of the chemical attacks. Dr. Muhammad, a doctor in Ghouta, said patients were coughing blood, a symptom not seen in previous chemical attacks.
Professor Rapharl Pitti of UOSSM France said that after viewing videos from the site, the patients seem to present convulsions more typical of sarin.
"Everything suggests that during the second attack, chlorine was used to conceal the use of sarin at the same time," he said.
The attack appears to have brought Assad closer to sealing victory in the military campaign for eastern Ghouta launched by the Russian-backed Syrian army in February.
Following the attack, the rebel group Jaish al-Islam gave way to a Syrian government demand that it accept safe passage out to rebel-held areas at the Turkish border.
It marks Assad's most significant victory over the rebellion against his rule since 2016, and underlines his unassailable position in the conflict.
Reporting by Nayera Abdallah, Suleiman Al-Khalidi, Ellen Francis, Maria Kiselyova, Dan Williams, John Irish, Matt Spetalnick; Writing by Angus MacSwan; Editing by Janet Lawrence.