The Syrian government and its ally Russia have blamed Israel for a deadly attack on a Syrian military airport.
Monday's attack hit the Tiyas airbase, known as T4, near the city of Homs. Observers say 14 people were killed.
Israel, which has previously hit Syrian targets, has not commented. Syria initially blamed the US for the strike.
The incident comes amid international alarm over an alleged chemical attack on a Syrian rebel-held town. The US and France had threatened to respond.
US President Donald Trump said there would be a "big price to pay" for the alleged chemical attack in Douma, in the Eastern Ghouta region, near the capital Damascus. He branded Syria's President Bashar al-Assad an "animal".
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
....to pay. Open area immediately for medical help and verification. Another humanitarian disaster for no reason whatsoever. SICK!— Donald J. Trump (@realDonaldTrump) April 8, 2018
But Russian Foreign Minister Sergei Lavrov said accusations that Mr Assad is behind the attack were a "provocation".
Moscow favoured an "honest investigation" of such incidents but opposed apportioning blame without any proof, he added.
Meanwhile, Ahmet Uzumcu, head of the Organisation for the Prohibition of Chemical Weapons, expressed "grave concern" about the alleged attack. The OPCW is currently gathering information about the possible use of chemical weapons.
Could Israel be involved?
Syrian state news agency Sana, quoting a military source, reported that air defences had repelled an Israeli missile attack on T4, saying the missiles were fired by Israeli F15 jets in Lebanese airspace.
Russia's defence ministry said that, of eight missiles, five were shot down and three reached the western part of the aerodrome.
Israel rarely acknowledges carrying out strikes, but has admitted attacking targets in Syria dozens of times since 2012. Its heaviest air strike on Syria, in February this year, included targeting the T4 air base.
That followed an incursion by an Iranian drone into Israel and the shooting down by Syrian air defences of an Israeli F16 fighter jet.
Israel has said it will not allow Iran, its arch-foe, to set up bases in Syria or operate from there, something Israel considers a major threat.
The Israeli military said Iran and its Revolutionary Guards had long been active in the T4 base, and were using it to transfer weapons, including to Lebanese Shia militant group Hezbollah, an enemy of Israel.
They also said the drone had been launched from the base.
UK-based monitoring group, the Syrian Observatory for Human Rights, said that fighters of various nationalities - meaning Iranians or members of Iranian-backed Shia militias - were among the 14 dead at the base.
By Jonathan Marcus, BBC defence and diplomatic correspondent
This attack could be part of Israel's growing effort to contain Iran's military build-up in Syria and to interrupt the supply of advanced Iranian missiles to its Lebanese Shia ally, Hezbollah.
Any Israeli operation would have been closely monitored by Russian air defence radars in Syria. There is also a telephone hotline between the Israelis and the Russian headquarters in Syria.
So far Moscow has done nothing to interfere with Israel's air operations.
But the presence of Russia's air defences in Syria certainly complicates the strategic equation as Western governments ponder their response to the recent chemical attack.
Is there a connection to the Douma attack?
Mr Trump and France's President Emmanuel Macron issued a joint statement on Sunday vowing to "co-ordinate a strong, joint response" to the alleged chemical attack.
But US officials said America did not launch the overnight strike.
"At this time, the Department of Defense is not conducting air strikes in Syria," the Pentagon said in a statement.
"However, we continue to closely watch the situation and support the ongoing diplomatic efforts to hold those who use chemical weapons, in Syria and otherwise, accountable."
France also issued a denial.
Sana initially called the reported missile strike on the Tiyas airfield a "suspected US attack", but later dropped the reference to the US.
In April 2017, the US fired 59 Tomahawk cruise missiles at Syria's Shayrat military airfield in response to a chemical weapons attack on a different rebel-held town, Khan Sheikhoun.
What happened in Douma?
Medical sources say dozens of people were killed on Saturday in the attack on Douma, in the Eastern Ghouta region.
One video, recorded by rescue workers known as the White Helmets, shows a number of men, women and children lying lifeless inside a house, many with foam at their mouths.
However, it has not been possible to verify independently what actually happened, or the actual number of dead.
The UN Security Council is expected to discuss the crisis on Monday.
Both Syria and Russia deny a chemical attack took place, and have reached an evacuation deal with the Jaish al-Islam rebels who hold Douma.
Moscow said military operations there had been halted. Under the deal, 100 buses are said to be moving 8,000 fighters and 40,000 of their relatives out of the battered town. Hostages who had been held by the rebels are being set free.
The development means pro-government forces have now taken full control of Eastern Ghouta.
Analysts say this is President Assad's biggest military success since the fall of Aleppo in 2016. It follows a weeks-long government offensive in which more than 1,600 people were killed.