Theresa May condemns 'barbaric attack' in Syria
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EPA
Theresa May has said she "utterly condemns" the "barbaric" alleged chemical weapons attack in Syria.
The PM said if the attack was confirmed as another example of President Bashar al-Assad's regime's brutality, "the regime and its backers including Russia, must be held to account".
Russia has said no evidence of a chemical weapons attack in formerly rebel-held Douma has been found.
The US and France threatened a "joint, strong response" to the alleged attack.
"The UK utterly condemns the use of chemical weapons in any circumstances and we must urgently establish what happened on Saturday," Mrs May said after a meeting with the Danish Prime Minister Lars Lokke Rasmussen in Copenhagen.
"If confirmed this is yet another example of the Assad regime's brutality and brazen disregard for its own people and for its legal obligations not to use these weapons.
"If they are found to be responsible, the regime and its backers - including Russia - must be held to account."
She said the UK was urgently working with its allies to assess what had taken place.
"We've seen it's not only adults that have been affected, but children have been affected by this attack," she said.
Medical sources say dozens were killed in Saturday's attack, but numbers are impossible to verify.
Russian Foreign Minister Sergei Lavrov had said no evidence of a chemical weapons attack had been found, saying Russian specialists and aid workers had visited the area.
The United Nations Security Council is to discuss the allegations later on Monday.
Mrs May said the events in Douma fit into "a troubling wider pattern of acts of aggression and abuse of long-standing international norms on counter proliferation and the use of chemical weapons".
She said Russia's "repeated vetoes" at the UN "have enabled these rules to be broken and removed mechanisms that allow us to investigate and hold to account chemical weapons attacks in Syria".
"This must stop," she said.
Last year, Russia repeatedly used its veto powers at the UN in support of its ally, blocking a UN Security Council resolution that would have extended an international inquiry into chemical weapons attacks in Syria.
Comparing the attack in Douma to the "similar recklessness" seen last month with the poisoning of a former Russian spy and his daughter in Salisbury, the prime minister said that together with the international community, the UK will not tolerate Russia's "illegal and destabilising activity" and its "increasingly hostile behaviour".
"The UK's case for holding Russia responsible for the attempted murder of Sergei and Yulia Skripal is clear," she said.
"The government has concluded there's no plausible explanation other than that Russia was responsible. No other country has a combination of the capability, the intent and the motive to carry out such an act."