David Cameron said on Friday that growing evidence of the use of chemical weapons by the Syrian regime was "extremely serious".
Britain's prime minister agreed with US President Barack Obama that such use would represent a "red line" for the international community, but said the response would likely be political rather than military.
"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," Cameron told the BBC.
"I've always been keen for us to do more."
He added: "The question is how do we step up the pressure.
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"In my view what we need to do -- and we're doing some of this already -- is shape that opposition, work with them, train them, mentor them, help them so we put the pressure on the regime and so we can bring this to an end."
Asked whether that would mean putting British troops on the ground in Syria, Cameron said: "I don't want to see that and I don't think that is likely to happen.
"But I think we can step up the pressure on the regime, work with our partners, work with the opposition in order to bring about the right outcome."
The United States said Thursday for the first time that Syria had likely used chemical weapons against rebel forces, but emphasised spy agencies were still not 100 percent sure of the assessment.
Britain's Foreign Office confirmed it also had "limited but persuasive" evidence of the use of chemical agents in the conflict which the UN says has left more than 70,000 dead since March 2011.
"It is limited evidence, but there is growing evidence that we've seen too of the use of chemical weapons, probably by the regime," Cameron said on Friday.
"It's extremely serious -- this is a war crime and we should take it very seriously."