Russian Foreign Minister Sergei Lavrov has said no evidence has been found of a chemical weapons attack in Syria’s formerly rebel-held town of Douma.
He said Russian specialists and aid workers had visited the area. Rebel fighters have started leaving the town under a surrender deal.
Medical sources say dozens were killed in Saturday’s alleged attack but numbers are impossible to verify.
The US and France threatened a “joint, strong response” to the alleged attack.
The United Nations Security Council is to discuss the allegations later on Monday.
The Russian denial came hours after a deadly attack on a Syrian military airport, which Moscow and the Syrian government blamed on Israel.
It 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 Washington for the strike, but the US, UK and France have all denied involvement.
What has been the reaction on Douma?
US President Donald Trump said there would be a “big price to pay” for the alleged attack in Douma, in the Eastern Ghouta region, near the capital Damascus. He branded Syria’s President Bashar al-Assad an “animal”.
Along with France’s President Emmanuel Macron, he issued a joint statement on Sunday vowing to “co-ordinate a strong, joint response”.
UK Prime Minister Theresa May’s spokesman said Britain was working with its allies on the response.
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.
What do the Russians say?
Mr Lavrov said that the Russian military had warned many times of a “provocation” being prepared, aimed at putting the blame on Damascus for the alleged use of chemical weapons against civilians.
“Our military specialists have visited this place, along with representatives of the Syrian Red Crescent… and they did not find any trace of chlorine or any other chemical substance used against civilians,” he said.
Moscow favoured an “honest investigation” of such incidents, he said, but opposed apportioning blame without any proof.
What is happening in Douma?
Medical sources say dozens of people were killed on Saturday in Douma.
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 number of dead.
Syria and Russia have reached an evacuation deal with the Jaish al-Islam rebels, who up until now have been holding 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 the 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.
What about the airfield attack?
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.
It said there were casualties, without giving a number.
Russia’s defence ministry said that, of eight missiles, five were shot down and three reached the western part of the aerodrome.
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.
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.