Media playback is unsupported on your device Media caption David Cameron: "What I see does look very much like a war crime is being committed"

UK PM David Cameron has expressed concern that international action in Syria may be being held back because of fears of a repeat of the Iraq war.

It follows evidence from the US and the UK that Syrian government troops may have used chemical weapons.

Mr Cameron said world leaders must look at Syria and "ask ourselves what more we can do."

Meanwhile, US President Barack Obama said proof the weapons had been used on civilians would be "a game changer".

The US president has pledged a "vigorous investigation" after US intelligence found Syria may had used the nerve agent sarin.

Mr Obama had said previously that chemical weapons use would cross a "red line" and provoke a major American response.

The UK government has also said it has "limited, but growing" evidence of the use of chemical weapons in the conflict.

Syrian officials have denounced the allegations as "lies".

'Proper processes'

Mr Cameron told the BBC he worried Western leaders might fail to act in Syria because of the experience of foreign intervention in Iraq.

Analysis The US, Britain, Israel and others have been collecting evidence to try and determine whether chemical weapons have been used in Syria. The signs so far are that they have been, but politicians are being cautious of over-selling their level of certainty. This is partly because of the lesson of Iraq, when too much was based on too little hard information and all the caveats and cautions surrounding intelligence were lost. And also partly because this time the political context is different. With Iraq a decision had been made to go to war and the intelligence was brought into the public domain to make the case for it. This time political leaders - especially in Washington - seem much more reluctant to intervene and so the emphasis is precisely on the caveats and cautions. Given the problems of getting access to a warzone to gain conclusive evidence, finding absolutely definitive evidence may be hard and take time. This may buy politicians in Washington and London time to work out what they do if something is found.

But he insisted lessons about acting on intelligence reports had been learned.

Speaking to the BBC's political editor Nick Robinson, 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."

The prime minister then addressed concerns about the quality of the UK's intelligence and fears that unreliable evidence could again be used as a justification for the West to become involved in a Middle Eastern conflict.

He said: "I would want to reassure people and say the lessons of Iraq have been learned.

"There are proper processes in place to try and make sure that what people say is properly backed up by the information.

"If anything, I would argue that because people are so worried about what happened in Iraq, it's actually quite important now to come forward - as the Americans have done and I think [US President] Barack Obama has done it in a very clear and measured way."

Mr Obama warned in December that Syrian President Bashar al-Assad would face "consequences" if he used chemical weapons.

Mr Cameron has said he agrees with the White House's warning that chemical weapons use would be a "red line", although he has ruled out sending British troops into action and has spoken instead of support for rebel forces.

However, former Liberal Democrat leader Sir Menzies Campbell has criticised what he called "red line diplomacy".

Sir Menzies said on the BBC's Any Questions that the Syria situation was "a very good argument against so-called red line diplomacy".

What is Sarin? One of a group of nerve gas agents invented by German scientists as part of Hitler's preparations for World War II

Huge secret stockpiles built up by superpowers during Cold War

20 times more deadly than cyanide: A drop the size of a pin-head can kill a person

Called "the poor man's atomic bomb" due to large number of people that can be killed by a small amount

Kills by crippling the nervous system through blocking the action of an enzyme that removes acetylcholine - a chemical that transmits signals down the nervous system

Can only be manufactured in a laboratory, but does not require very sophisticated equipment

Very dangerous to manufacture. Contains four main ingredients, including phosphorus trichloride Syria's chemical weapons stockpile How to investigate allegations

"It encourages your adversary to go as close to the red line as he can possibly manage... and you are prejudging the circumstances which might be months or even years down the line."

Libya

Asked directly if he was concerned the invasion of Iraq in 2003, which toppled Saddam Hussein, was having an impact on the way in which Western leaders were dealing with the conflict in Syria, Mr Cameron said: "I do worry about that."

"Let me absolutely clear, I think the Iraq lesson must be about how we marshal and use information and intelligence and I think that lesson has been learnt - but I think it is very important for politicians and leaders of this generation to look at what is happening in Syria and ask ourselves what more we can do."

He said that before any action was taken in Syria it was necessary to ensure you could "achieve the result you want".

"The reason we could act in Libya was because we had an opportunity, if we acted quickly, to stop a dictator in his tracks. We could do that. It is very important that the ability is there and we have to think about that carefully."

On Friday, the White House insisted there was no timeline on how long it would take to corroborate the evidence on the use of chemical weapons and admitted that the case was "not airtight".

But spokesman Jay Carney repeated that all options remained on the table.

Media playback is unsupported on your device Media caption Times journalist Anthony Loyd visited victims of a suspected chemical attack in hospital

Earlier on Friday, Syrian official Sharif Shehadeh told the Associated Press the US allegations of chemical weapons use were "lies".

Syria's Deputy Foreign Minister Faisal Mekdad also dismissed the accusations in an interview with Reuters.

Syria's government and rebels have accused each other of using chemical weapons. A UN team is trying to enter Syria to investigate.