WASHINGTON, May 21: Officials have no specific credible evidence of a threat of suicide bombings in the United States, but Americans would be “somewhat naive” if they felt immune from such attacks, Homeland Security chief Tom Ridge said on Tuesday.

“While we prepare for another terrorist attack, we need to understand that it is not a question of if, but a question of when,” Ridge told business executives a day after FBI Director Robert Mueller warned it was “inevitable” that suicide bombers would hit the United States in the future.

Ridge, joining a chorus of top officials warning about possible fresh attacks after Sept. 11, said Mueller’s remarks were not based on specific information that suicide bombings were imminent, but the Bush administration was committed to sharing as much information with the public as it could.

“I think the FBI director was speaking to the reality of the wide range of potential terrorist attacks that we have witnessed in other parts of the world,” Ridge told Reuters after a speech to the World Economic Forum.

“He was just speaking to the reality that we would be somewhat naive if we felt that terrorists would limit the use of that tactic to other parts of the world, but not the United States,” he said.

“He was reminding this country that we should not feel that we are immune from terrorists using that kind of tactic against us as well,” he said.

Ridge told the conference the United States remained on an “elevated” level of risk, or yellow ranking, on a color-coded national alert system introduced in March, because intelligence on possible attacks was too vague. The highest alert level is red, followed by orange, yellow, blue and green.

“It wasn’t actionable in the sense that we’re going to change a national level of alert, but it was informational,” Ridge said, speaking about warnings of possible fresh attacks.

Top Bush administration officials — under fire for their handling of intelligence before the September attacks — have stepped up their warnings in recent days that extremists could launch new attacks after killing more than 3,000 people on Sept. 11.

Vice President Dick Cheney said on Sunday extremists were “almost certain” to strike again, and intelligence officials cited signs a new al Qaeda strike could be in the offing. Osama bin Laden’s al Qaeda network is blamed by Washington for the Sept. 11 attacks.

Mueller told a closed-door meeting of the National Association of District Attorneys on Monday it was “inevitable” that suicide bombers like those who have killed dozens of Israelis in restaurants and buses would strike the United States, a Justice Department official said.—Reuters