David Cameron has virtually ruled out sending British troops into Syria after growing evidence chemical weapons have been used against rebels in the civil war.

The Prime Minister, while keen to avoid an Iraq-style invasion, warned that more effort was needed by the international community to tackle Bashar al Assad's regime after tests at the Government's top secret Porton Down laboratories suggested that toxic substances had been deployed.

He said that although the evidence was so far limited, to use such weapons was an extremely serious development.

Mr Cameron said: "I choose my words carefully, but what I see does look very much like a war crime is being committed in our world, at this time, by the Syrian government."

US president Barack Obama has previously said the use of chemical weapons 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.

It confirmed US intelligence services had assessed with "varying degrees of confidence" that chemical weapons, believed to include the deadly nerve agent sarin, had been used by the regime on a "small scale".

Samples smuggled out of Syria were analysed by the Defence Science and Technology Laboratory at Porton Down in Wiltshire.

Labour leader Ed Miliband said the use of chemical weapons was appalling and "would necessitate some kind of action", but urged caution.

He added: "The question is what action is possible? That's what the international community has got to work on, and then the most important thing for the international community, which frankly so far it has failed to do, is to show some unity."

Bernard Jenkin, the Conservative backbencher and a former party defence spokesman, said he was sceptical about the benefits of foreign intervention. He added: "But we appear to be making the same mistakes that we made in Bosnia and the former Yugoslavia, where we allowed a problem to fester and become worse and worse."

London and Washington have called on Mr Assad, the Syrian President, to allow United Nations inspectors to carry out a full investigation to establish what has happened.

However, Sharif Shehadeh, a Syrian official, dismissed the allegations of chemical weapons use by the Syrian government, branding them "lies".

Mr Cameron stressed western leaders were trying not to make the mistake of "rushing into print" and were working to consider and verify the evidence.

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 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."