Top alert in US on Independence Day

America is preparing for its most anxious Fourth of July ever, amid fears that terrorists could use the Independence Day celebrations to launch a much-anticipated strike.

Last week, the Pentagon revealed the military will be stepping up random air patrols over key US American cities on and around 4 July as part of increased security measures. In addition, the Federal Aviation Administration has announced new restrictions on flights around a series of popular US landmarks, including the Statue of Liberty, Mount Rushmore and the Gateway Arch in St Louis.

"This is not in response to any specific threat," one senior defence official was quoted as saying last week. "It's more a recognition that it is a day of symbolic significance." But a stream of warnings and alerts, such as one that terrorists might use petrol tankers to strike at Jewish targets, have sometimes given the impression that Americans are jumping at shadows.

While officials are keen to play down the threat, President George Bush yesterday highlighted the anxiety that is gripping many in the US when he transferred power to Vice-President Dick Cheney for two hours while he underwent a routine colonoscopy that required him to be sedated. It was understood that Mr Bush was not fully unconscious during the procedure, but the President said he was being "super-cautious" because of his "war against terror".

"I did so because we're at war and I just want to be – you know – super cautious," Mr Bush said on Friday evening. "It is a normal procedure, but these are not normal times."

Officials voiced concern about the possibility of an attack on 4 July some weeks ago. It is understood that many of the warnings in recent months have been based on information provided by the senior al-Qa'ida leader, Abu Zubeida, who is currently being interrogated by the US at an undisclosed location.

Specific concerns about July were heightened when the FBI revealed they were investigating the claims of a Lebanese-born man who said he had overheard a mobile phone conversation in Arabic talking of a possible 4 July attack on Las Vegas.

Michael Hamdan, a Las Vegas businessman, said he heard the speakers discussing a "hit" on the "day of freedom".

The general level of anxiety has not been eased by recent comments by Mr Cheney and the Defence Secretary, Donald Rumsfeld, who have both essentially said it was a matter of "when, not if" terrorists would try and launch another attack on the US.

Last week Mr Rumsfeld refused to comment about current concerns, saying: "I'm not in the business of giving warnings or alerts or a heads-up with respect to possible terrorist events in the United States. We're constantly adjusting force levels and threat levels in various parts of the world based on intelligence, but I have nothing I'd really want to say about that."

The 7,000 US forces in Afghanistan will be on a heightened state of alert on Thursday. At Bagram airfield, the US headquarters, guards at the perimeter of the base have been told to be extra vigilant. "It's the Fourth of July coming up, and we need to go on a heightened state of security," said spokesman Colonel Roger King.

The alert will not prevent some 4 July celebrations at the base, however. Some soldiers will participate in a six-mile run, followed by a small picnic. Others plan to mark the holiday by re-enlisting.