AMERICA is on high alert this weekend as the 10th anniversary of the Twin Towers terror attack that changed the world is marked with sadness and trepidation.

Hillary Clinton, the US Secretary of State, called for vigilance after she revealed there had been a “specific, credible but unconfirmed report that al Qaeda again is seeking to harm Americans and, in particular, to target New York and Washington”.

On the back of intelligence from Pakistan’s tribal areas, US police are pursuing three individuals, said to be either travelling to America or who had arrived there recently with the intention of blowing up car and truck bombs in tunnels or on bridges.

Security, already heightened for the 9/11 anniversary, particularly at airports, railway stations and border crossings, has been further enhanced. Armed guards were stationed at New York landmarks including Wall Street, Times Square and the memorial site where the Twin Towers once stood.

“We’re hyper-vigilant to this specific report that’s just coming in,” said Jay Carney, the White House spokesman.

At home, the Foreign Office issued its own alert to Britons travelling to or living in America while Strathclyde Police sought to reassure Scots by saying: “Sufficient resources will be available over this weekend as the world remembers the tragic events of 9/11.”

The White House made clear there were no plans to change the president’s schedule, as he is due to mark the anniversary with visits to Ground Zero, the Pentagon and Shanksville, Pennsylvania, on Sunday.

Michael Bloomberg, the Mayor of New York, said police were deploying extra officers. He urged New Yorkers to go about their business as usual but added: “We should all keep our eyes wide open.”

Cathy Lanier, the city’s police chief, announced enhanced security measures. These included vehicle check-points, more bag inspections on the subway, extra sniffer dogs and more radiation monitoring equipment. Unattended cars parked in unusual locations risked being towed, she added.

Police Commissioner Ray Kelly also said: “There will be increased focus on tunnels and bridges and infrastructure in general as well as landmark locations, houses of worship and government buildings.”

Information gleaned by US special forces when they raided Osama bin Laden’s compound in Pakistan earlier this year had already shown that al Qaeda was considering attacking America again on the 9/11 anniversary as well as other important US dates. Intelligence chiefs believe such a possibility might have increased as the terror group seeks revenge for its leader’s death.

In London, the Foreign Office said: “There is a general threat from terrorism in the United States. Attacks could be indiscriminate, including in places frequented by expatriates and foreign travellers. In light of the publicity surrounding the anniversary of the 9/11 attacks, we remind British nationals of the global threat of terrorism and the need to remain vigilant.”

Whitehall officials made clear there had been no change to the assessment of the threat level posed to the UK by international terrorism, which, in July, was reduced from “severe” to “substantial”, which means an attack is considered a “strong possibility”.

In Glasgow, Strathclyde Police Assistant Chief Constable George Hamilton, said: “It is not appropriate to publicly declare resourcing levels dedicated to countering terrorism as this would be counter-productive. However, it is our job to keep people safe and we want to reassure people that sufficient resources will be available over this weekend.”

Ten years ago tomorrow, the world looked on in horror as al Qaeda terrorists hijacked planes and flew them into the Twin Towers of the World Trade Centre. Nearly 3000 people were killed, including 67 Britons. The attack led directly to the allied war against the Taliban, who were allowing al Qaeda to train terrorists in Afghanistan.

In a broadcast with al Jazeera TV

last night, David Cameron said al Qaeda was an organisation that “at its heart is committed to mass murder”.

He stressed that while one of the lessons from 9/11 was to deal with grievances in the Muslim world, another was to show strength when faced with terror.

He added: “What you’ve seen, in terms of the senior leadership of al Qaeda and the end of bin Laden, is hugely encouraging in that this poisonous organisation that has killed so many people in so many different countries -- and remember, they probably killed more Muslims than any other religious group -- has come under tremendous pressure and that’s a very good thing.”

Meantime, George W Bush, whose presidency was defined by the terrorist attacks in 2001, said the hunt for bin Laden “began the day after 9/11”.

The former US President revealed that he was concerned America was becoming “isolationist”. He said: “What happens overseas can affect us here at home and, therefore, the US ought to be engaged in the world.”

Mr Bush admitted that, except for his wedding and his daughters’ births, 9/11 was “the most profound moment” of his life. “It will always be vivid,” he added.

Iain MacWhirter