The "limited but growing evidence" that chemical weapons have been used in Syria's civil war must spur the international community into doing more, David Cameron has said.
The Prime Minister described United States intelligence reports pointing to their use as "extremely serious" and said he had "always been keen for us to do more".
But he insisted a plan to put boots on the ground was unlikely.
"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," Mr Cameron told the BBC.
"It is extremely serious, this is a war crime, and we should take it very seriously."
US president Barack Obama has previously said the use of chemical weapons in Syria would be a "red line" but, following the experience of Iraq, the White House is proceeding cautiously to ensure it is not drawn into action on the basis of intelligence that could turn out to be faulty.
Both London and Washington have called on Syrian president Bashar Assad to allow United Nations inspectors to carry out a full investigation to establish what had happened. Mr Cameron said they were trying not to make the mistake of "rushing into print" and were working to consider and verify the evidence with Britain's allies.
"But 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.
"I have always been keen for us to do more. We are working with the opposition, we want our allies and partners to do more with us to shape that opposition to make sure we are supporting people with good motives who want a good outcome, to put pressure on that regime so we can bring it to an end."
Asked if there could be troops on the ground in Syria, Mr 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."