The Conservative Party leader, Iain Duncan Smith, has warned there must be no safe havens for those conducting terrorist attacks.

Speaking at a conference on terrorism on Friday, Duncan Smith told an American audience that he gives his full backing to the "war on terrorism".

He said: "My message is simple - we are in this together."

Describing the removal of the Taliban regime in Afghanistan as "now virtually assured", Duncan Smith warned Osama bin Laden that "the net is closing in".

"Yet, while the war in Afghanistan might just be beginning to have an end in sight, the war against terrorism is emphatically not over. It must go on," he told the American Enterprise Institute in Washington DC.

He set out a tough message for other "rogue states" that may be harbouring those committing acts of terrorism - warning the leaders of such regimes that "the days of the safe havens are over".

"We are no longer prepared to tolerate your activities. That goes for Afghanistan, just as it should for other countries we know, and can show, are involved in international terrorism. Where these states are unwilling to take effective action against terrorism they must be prepared to face a determined response from the wider international community - and I hope that the United Kingdom will continue to be at the forefront of that response," he said.

Duncan Smith reiterated his strong backing for the Bush administration's plans for a ballistic missile defence shield, describing it as "essential for us to look collectively at new ways of strengthening our defences".

"Far from holding back on missile defence, I believe that the events of September 11 have made it all the more important to press ahead," he argued.