The UK and its allies may need to take "further steps" against Bashar Assad's regime if it is proved that chemical weapons have continued to be used in the Syrian civil war, David Cameron said.
The Prime Minister told MPs that reports that chlorine had been used in recent months were "extremely disturbing" and if they were shown to be true then the UK would decide on "appropriate action".
Mr Cameron also warned that Britons being radicalised while taking part in the Syrian conflict could be the greatest terrorist threat facing the country.
The Prime Minister suffered a Commons defeat last year over plans which could have resulted in British military intervention against Assad's regime over the use of chemical weapons.
In the wake of the vote, and with US military action still a possibility, a Russian-brokered deal saw Assad agree to give up his stockpiles of the weapons.
Mr Cameron said that around 90% of Assad's chemical weapons stockpile had now been removed, but they were behind on their deadlines.
He said that the issue would be pursued "very vigorously" and, if the use of chlorine was proved, "we need to think of what further steps that we can take because it is completely unacceptable to use chemical weapons in the world we live in today".
Appearing before the Liaison Committee of senior MPs, he said: "The reports are extremely disturbing about whether they are being used. We want those checked out and then we should be working with our allies to determine what would be the appropriate action to take."
He added: "We have to get straight what has happened on the chemical weapons front. I have been very clear, obviously the steps I was contemplating taking we weren't able to take, but I was very clear we needed clear evidence before taking any steps and we should always deal with these things on the basis of evidence."
Foreign Affairs Committee chairman Sir Richard Ottaway asked him whether there should be a fresh look at the issue: "If the situation is getting to the state whereby chlorine is being used contrary to international conventions, isn't it time to revisit the issue?"
Mr Cameron said: " I think the mood of Parliament was pretty clear that it did not want to take part in any military action in respect of chemical weapons use.
"So I think the right thing to do is to gather evidence of any use, be absolutely clear about what has happened, then work out with our allies what the right response is.
"And if anything changes then to come back to Parliament and explain it."
The Prime Minister restated his view that the UK should offer non-lethal assistance to opposition groups but should not arm them.
He added: "Our view is very clear that Assad can't be part of Syria's future. The bloodshed, the things that he has visited onto his own people, makes it impossible to envisage a united, democratic, free Syria in which he plays a part."
Mr Cameron acknowledged that in recent months Assad has had some "tactical successes but it's hard to go further than that when you see the extent of the bloodshed in the country, the extent of the division, the fact that he is not in control of most of it".
Hundreds of Britons are believed to have travelled to Syria to take part in the conflict and Mr Cameron said they could pose a risk on their return.
The Government's aim was to prevent people from going in the first place to avoid the risk of them becoming radicalised, the Prime Minister said.
"We need mums and dads to help, we need schools to help, we need communities to recognise the importance of this. The Government funds programmes with steering people away from radicalisation.
"But then there is the harder end of things: we do have to make sure that our police and security services are all over people thinking of travelling to Syria and are stopping them - taking away their passports, taking away their nationality if necessary, preventing them from going, perhaps preventing them from coming back if they have gone and making sure we do everything within our power to keep us safe.
"There is no doubt this is now, along with the federally administered tribal areas of Pakistan and Afghanistan, Syria is now as big a problem - if not a bigger problem - in terms of counter-terrorism, in terms of the threat to our country."
The Prime Minister also said he would put pressure on the French to increase their aid programme for Syria.
The UK has committed around £600 million to the relief effort in Syria, and has a "very proud record", Mr Cameron said.
"If you look at what Kuwait is giving or the United Arab Emirates, they are making significant contributions, the Germans so far have given about 560 million dollars (£333 million).
"That is a big contrast to the French figure of 75 million (£45 million) or the Italian figure of 91 million (£54 million)."