US Vice President Dick Cheney said yesterday that a new attack on the United States was "almost certain" as US intelligence officials picked up signals that a fresh strike could be in the works.

Speaking in two television interviews, Cheney also sought to blunt criticism the Bush administration failed to pick up hints last summer that critics believe might have helped prevent the September 11 attacks and he tried to brace the country for any new assault.

"In my opinion the prospects of a future attack against the United States are almost certain," Cheney said on NBC`s Meet the Press programme. "It`s not a matter of if, but when."

A White House official on Saturday said US intelligence officials have detected "enhanced activity" that points to a potential new attack against the United States or American interests abroad.

The FBI also warned of a possible plot by Saudi-born militant Osama bin Laden`s al Qaeda network, which the United States believes carried out the September 11 attacks, to detonate bombs in US apartment buildings.

The comments came as The New York Times reported US intelligence agencies had intercepted a series of messages among al Qaeda operatives indicating the group is attempting to launch an attack as big as or bigger than the one on September 11.

Quoting unidentified intelligence and law enforcement officials, the Times characterised the communications as vague but disturbing.

The intercepted messages are so general that they have left US President George W. Bush and US counterterrorism officials uncertain about the timing, location or method in this potential attack, the Times reported.

Yesterday, Cheney acknowledged the US government had failed to anticipate the September 11 attacks, which killed around 3,000 people in New York, Washington and Pennsylvania.

"There`s no question but what there were failures. We obviously did not know what was about to happen. We were unable to prevent it," he told Meet the Press, saying the government did not adequately coordinate information from its intelligence services and its domestic law enforcement agencies.

But Cheney also voiced anger at what Bush has called "second-guessing" by Democrats who last week pounced on the disclosure that the president had received an August 6 briefing evoking the possibility of al Qaeda hijacking.

Some Democrats and outside critics have suggested the White House failed to piece together a string of hints that, in retrospect, appeared to presage the attacks on America.

These include a 1999 report written for the CIA that suggested bin Laden-linked suicide bombers might crash a plane packed with explosives into the Pentagon, the CIA or the White House, and a memo written last July by an FBI agent urging his superiors to investigate Middle Eastern men enrolled in American flight schools.

"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," Cheney said, adding that he did not want to see the disclosure of classified information that could make it harder to foil any future attacks.

Speaking on the "Fox News Sunday" program, Cheney said it was virtually impossible to create a fool-proof defense against attacks, noting that Israel - with a vaunted intelligence service and a much smaller geographic area to protect - had not been able to prevent a string of suicide bombings.

The New York Daily News reported yesterday that five New York Police Department investigators are currently in Israel for training to detect and investigate suicide bombers.

"It`s almost impossible to erect a 100 per cent perfect defense," Cheney told "Fox News Sunday," noting that bin Laden in a videotape had suggested some of the September 11 hijackers did not know they were on a suicide mission.

"If they can keep the people who actually are carrying out the attack in the dark, then obviously the extent to which you can get access to find out the details of the operation is very limited and it`s very hard," Cheney said.

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