Syria on Wednesday missed a deadline to hand over all the toxic materials it declared to the world's chemical weapons watchdog, putting the programme several weeks behind and jeopardising a final June 30 deadline.
Cameron, who last August lost a parliamentary vote to authorise British participation in possible air strikes on Syria, was responding to a question from a senior lawmaker.
Cameron said he shared growing anxiety among British parliamentarians that the Syrian programme had fallen "so badly behind".
"There do seem to be now indications that the programme is slowing and that not all the information necessary is forthcoming," Cameron told parliament.
"Britain will continue to put pressure on all parties to make sure the chemicals weapons are produced and destroyed."