David Cameron has said there is growing evidence of the use of chemical weapons by Bashar al Assad's regime, condemning the reported attacks as a war crime.
However, he said the latest developments did not mean Britain would send troops into Syria, but urged for more pressure to be put on the Assad regime.
His comments came as footage emerged of victims of an apparent sarin attack in the city of Aleppo.
The video, which was taken in a hospital and shows men and women frothing at the mouth and twitching from the effects of the nerve agent, was posted online just over a week ago.
The images were recorded at the medical facility in Afrin, about an hour’s drive from the city of Aleppo, where the alleged sarin attacks by government forces were carried out.
Dr Kawa Hassan, an orthopaedic surgeon who treated the first casualties, told Anthony Lloyd, a journalist for The Times who travelled to the medical facility: "We received an initial five casualties, then a second group.
"Many were foaming at the mouth and their pupils were dilated. Then some of my medical staff started to become affected, too. We gave them all atropine. For most, it had an immediate positive effect."
Mr Cameron told BBC Breakfast: "It is very disturbing what we are seeing. It's limited evidence, but there's growing evidence that we have seen too of the use of chemical weapons, probably by the regime.
"It is extremely serious, this is a war crime, and we should take it very seriously."
He said it was now essential to gather further evidence and for Britain to work with the international community to put pressure on the Assad regime.
He added: "This is extremely serious, and I think what President Obama said was absolutely right - that this should form for the international community a red line for us to do more.
However on Friday the White House was not ready to definitively rule chemical weapons had been used, saying it was "not an airtight case" against the Assad regime.
Syrian officials denied the government had used chemical weapons against rebel forces.
Sharif Shehadeh called the US claims "lies" and likened them to false accusations that Iraq possessed weapons of mass destruction.
US Defence Secretary Chuck Hagel announced during a trip to Abu Dhabi that evidence of chemical weapons use had been found.
His comments were backed by the US Secretary of State John Kerry, who said the Syrian regime had carried out two small-scale chemical weapons attacks.
Mr Kerry made the claim as he left a Congressional hearing, but did not give further details.
The White House said forces loyal to Assad probably used sarin gas against rebel fighters on a "small scale", but emphasised more assessments were needed.
Barack Obama has repeatedly said that any use of chemical weapons would be cross a "red line", triggering possible military action.
He is expected to discuss the matter when he meets King Abdullah II of Jordan later today.
The latest disclosures by those in his administration have led to calls for tough action including enforcing a no fly zone and creating safe zones inside the country.
Speaking yesterday, a White House official said if it was determined that line had been crossed, the US would consult with its allies on the next step - and "all options are on the table".
The official said the Assad regime is believed to have chemical weapons, and any use of them in the country would probably have been carried out by government forces.
Miguel E Rodriguez, assistant to the president and director of the Office of Legislative Affairs, revealed the claims in a letter to senators John McCain and Carl Levin.
"Our intelligence community does assess with varying degrees of confidence that the Syrian regime has used chemical weapons on a small scale in Syria, specifically the chemical agent sarin," he said.
A senior defence official, speaking on condition of anonymity, told the Associated Press the letter was not an "automatic trigger" for policy decisions on the use of military force.
Senator McCain told Sky News: "The problem is that the president has consistently said that's a red line, so the question is, will the president act in a way that I have advocated for a long time?
"(This involves) providing a safe zone for the Syrian resistance, provide proper weapons and have operational capability to secure these chemical weapon caches."
UN Secretary General Ban Ki Moon announced earlier this month that he had a team of 15 inspectors on stand-by in Cyprus ready to go and investigate allegations of chemical weapons us.
Today his spokesman Martin Nesirky said: "The Secretary-General has consistently urged the Syrian authorities to provide full and unfettered access to the team. He renews this urgent call today.
"The fact-finding team is on stand-by and ready to deploy in 24-48 hours."
Sarin is a colourless and odourless liquid, and is an extremely potent nerve agent.
It is classified as a weapon of mass destruction by the UN, and the production and stockpiling of sarin was outlawed by the Chemical Weapons Convention of 1993.