Islamabad ? Dozens of Muslim militants with European citizenship are believed to be hiding out in the lawless tribal area of northwestern Pakistan, Pakistani and Western intelligence officials say, training for missions that could include terror attacks in European capitals.

Officials have used phone intercepts and voice tracking software to track militants with ties to Britain and other European countries to areas along the Afghan border. Al-Qaida would likely turn to such extremists for a European plot because they can move freely in and out of Western cities.

Fear that such an attack is in the planning stage has prompted the U.S. State Department to advise Americans traveling in Europe to be vigilant. American and European security experts have been concerned that terrorists based in Pakistan may be plotting attacks in Europe with assault weapons, similar to the deadly 2008 shooting spree in Mumbai, India. U.S. intelligence officials believe Osama bin Laden is behind the plots.

A senior official of Pakistan’s Inter-Services Intelligence agency, or ISI, told The Associated Press that there are believed to be “several dozen” people with European citizenship — many of Pakistani origin — among the Islamic extremists operating in the lawless border area.

The official, who spoke on condition of anonymity because he was not supposed to talk about classified information to the media, said foreigners in the area also include Chechens, Uzbeks, Arabs and Turks, one of whom was a former F-16 pilot in the Turkish air force.

“That shows you that some of the people who are coming are very well educated,” he said. “It was very surprising for us but they come thinking this is the pure (Islamic) ideology that they are seeking.”

Britain’s communications monitoring agency, the Government Communications Headquarters or GCHQ, estimates there are as many as 20 British-born militants in the border area, especially in the North Waziristan district that has been the focus of recent missile strikes carried out by unmanned aircraft operated by the CIA.

Mobile phone communications have been tracked from the border area to points in Britain, particularly England’s Midlands, where there is a heavy Pakistani immigrant population, according to a British government official who spoke on condition of anonymity because the terror plot investigation is ongoing.