Michael Chertoff told the BBC that the US increasingly saw Europe as a "platform" for a terrorist attack and he blamed the current visa waiver programme for European citizens as a reason for America's vulnerability.

His remarks came as the BBC reported a posting to a known Islamic extremist website which talked about founding a branch of al-Qaida in the UK.

The posting, which was taken down after a day earlier this month, also talked about killing British politicians, including Gordon Brown.

In an interview recorded before that report, Chertoff said: "We have watched the rise of home-grown terrorism. We are obviously mindful of the Madrid bombings and the attempted bombings in Germany, and that suggests to us that the terrorists are increasingly looking to Europe as both a target and a platform for terrorist attacks."

He added: "When I lift my eyes and I look around the world and I look at what happens in Britain, in Germany, in Spain, in Bali, in Pakistan, I don't see terrorism going away. I see an al-Qaida that's evolving."

Chertoff also explained his unease about lax border control arrangements for Europeans.

"We have the visa waiver programme which allows most Europeans who come to be tourists to come without visas. That means the first time we encounter them is when they arrive in the United States, and that creates a very small window of opportunity to check them out," he said.

New travel restrictions were signalled by Chertoff. "What we are trying to do is find a way to better vet people coming in from Europe without impeding the flow of travel or trade which has been a very important part of our economy," he said.

Last May, the New York Times reported that Chertoff wanted to impose restrictions on British citizens of Pakistani origin.

It claimed he had suggested to the British government that British Pakistanis would have to apply for visas before travelling to the US.

At the time, the Foreign Office said it would resist the controversial idea.