The head of the Federal Bureau of Investigation says he thinks it inevitable that individual suicide bombers will begin attacks in America, in the way they do in Israel.

FBI director Robert Mueller said he wished he could be more optimistic but he felt that suicide attacks on public places would come eventually.

Cheney: Not a matter of if but when

Due to the fanaticism of groups such as Osama Bin Laden's al-Qaeda network, it was difficult for undercover agents to obtain access to information that would prevent such attacks, Mr Mueller was quoted as saying.

He was speaking a day after US Vice President Dick Cheney said new attacks on America by al-Qaeda are almost certain.

There has also been a warning from the chairman of the Senate Intelligence Committee that the US is potentially open to attacks from other extremist groups, not specifically al-Qaeda.

Senator Bob Graham told American news channel NBC that the country's enemy "is not al-Qaeda alone", and mentioned several prominent terror groups such as Hezbollah and Islamic Jihad that had the capacity - and the desire - to attack the US.

"There are several international terrorist groups which have abilities, in some cases greater abilities than al-Qaeda, and a similar desire to attack the United States," he said.

'Some success'

Such remarks from prominent members of the US intelligence community are an indication of increasing gloom within the American intelligence community, already under fire for failing to spot - and possibly prevent - the 11 September attacks.

I still have a deep sense of anger that anyone would suggest that the president of the United States had advance knowledge that he failed to act on

US Vice President Dick Cheney

On Sunday, US Vice President Dick Cheney also said that it was "almost certain" that al-Qaeda would carry out another terror attack on America.

It was "not a matter of if, but when" the militants blamed for 11 September would strike again, Mr Cheney said.

US security sources have indicated that al-Qaeda may be planning new attacks, and that these might involve planting bombs in apartment buildings.

However Mr Cheney defended the Bush administration's handling of reports on al-Qaeda activity prior to the September attacks, while acknowledging that intelligence agencies had failed to pool their information.

"We don't know if it's going to be tomorrow or next week or next year," said the vice president, adding that the US had had "some success" in disrupting al-Qaeda's network.

The militants, led by Osama Bin Laden, are believed to have organised the suicide attacks on 11 September, which killed about 3,000 people in New York, Washington, and Pennsylvania.

US National Security Advisor Condoleezza Rice said on Sunday that the US was on a far higher state of terror alert now than it was before those attacks.

'Chatter in the system'

Intelligence specialists have said there has been an increase in the volume of messages between al-Qaeda cells, similar to that noted before 11 September.

The Karachi bomb killed 14 people, including 11 French defence workers

An FBI spokeswoman, Debbie Weierman, said members of the network had reportedly been "considering renting apartments in unspecified areas of the United States and then planting explosives".

A senior Bush administration official quoted by the New York Times suggested that the new intelligence reports had yet to form a coherent picture.

"There's just a lot of chatter in the system again," the official said.

But a recent suicide bombing in Karachi, Pakistan, and an attack on a synagogue in Tunisia are seen as evidence by US intelligence officials that al-Qaeda still has the potential to stage deadly attacks.