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.
When asked whether she would recall Parliament over Syria, Mrs May said: "We are working urgently with our allies to assess what has happened and we are also working with our allies on what action might be necessary."
Medical sources say dozens were killed in Saturday's attack, but numbers are impossible to verify. Videos shot by rescue workers showed lifeless bodies of men, women and children with foam at their mouths.
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.
Russia - a key supporter of the Syrian government - has intervened militarily in the country. It launched an air campaign in support of Mr Assad in 2015 that was crucial in turning the war in the government's favour.
The United Nations Security Council met on Monday to discuss the allegations, which saw the US and Russia trade barbs.
Russian representative Vassily Nebenzia said the incident in Douma was staged and that US military action in response could have "grave repercussions".
But US envoy Nikki Haley said Russia - a Syrian military backer - had the "blood of Syrian children" on its hands.
On a visit to Denmark and then Sweden, 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.
Foreign Secretary Boris Johnson spoke to acting US Secretary of State John Sullivan on Monday about the incident, agreeing that it "bore hallmarks of previous chemical weapons attacks by the Assad regime", a statement from the Foreign Office said.
US President Donald Trump has said that the situation was being assessed and "major decisions" would be made within 48 hours.
Former Foreign Secretary William Hague has called for Mrs May to stand alongside the US, writing in the Daily Telegraph that chemical weapons will become "legitimised" and used in future wars if the West fails to take military action.
By John Pienaar, BBC News deputy political editor
Theresa May's flying Scandinavian tour - Denmark and Sweden in less than 24 hours - was supposed to be routine. As if...!
The prime minister's meetings with her counterparts in Copenhagen and Stockholm were wholly overshadowed by the alleged chemical attack in Syria: a challenge and a test not only for the Western powers and their allies, but for British global influence and for Mrs May herself.
As she met the Danish leader, Lars Rasmussen, in Copenhagen, it was clear she was keen to keep in step with key allies like America.
Tom Tugendhat, Tory chairman of the Commons Foreign Affairs Select Committee, told the BBC that the "UK should explore all options" and that is "absolutely right that we respond" to the attack in Syria.
He said the evidence "points squarely to the Assad regime" and that the "UK government has the legal authority" to respond.
Labour leader Jeremy Corbyn also condemned the use of chemical or biological weapons "in any scenario, anywhere in the world".
"The tragedy and the terror of people's lives in Syria can only end by a political solution," he said.
"That means every country in the region - every country in the region - as well as Russia and the United States coming together to ensure there is a meaningful ceasefire and there is a political process to bring about a political solution to the terror and tragedy and conflict that has wasted so many lives in Syria."
Shadow foreign secretary Emily Thornberry said: "What has happened in Douma clearly looks to be just the latest abhorrent attack in Syria using chemical weapons, a war crime for which the Assad regime has been found responsible in the past and which we utterly condemn."
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 added that together with the international community, the UK would not tolerate Russia's "increasingly hostile behaviour".