|Intelligence sources say an al-Qaeda plot for simultaneous attacks in cities across Europe have been foiled [EPA]
The United States has issued a warning to American citizens to be vigilant as they travel in Europe in light of intelligence pointing to possible attacks by al-Qaeda in Britain, France and Germany, officials have said.
The State Department said on Sunday that public transportation systems and other tourism-related facilities in the continent could be targets.
"The State Department alerts US citizens to the potential for terrorist attacks in Europe," the statement said.
"US citizens should take every precaution to be aware of their surroundings and to adopt appropriate safety measures to protect themselves when traveling," it warned.
The alert also said attackers may use "a variety of means and weapons and target both official and private interests," and that particular targets could be railways, subways and other tourist infrastructure.
The statement said "current information suggests" al-Qaeda and affiliated groups were continuing to plan attacks there.
Al Jazeera's Paul Brennan, reporting from London, said: "The US State Department has been aware of the growing concerns of the intelligence that has been growing pace over the past couple of days about the prospect of terrorist attacks.
"What is interesting is the backdrop of this statement. It is clear that European intelligence agencies are still very concerned about details that have been coming out from Bagram air base (in Afghanistan), where it is known they have been interrogating who they call a high value suspect: A man called Ahmed Siddiqui.
"He is a German man of Pakistani origin who is said to have had details of plans by Al Qaeda to launch Mumbai style attacks. That is small cells of highly mobile and armed people in European cities."
As a result of the warning from the State Department, teams from the US National Basketball Association that are in Europe have promised to take "appropriate" security measures as they continue their pre-season activities in the continent.
The Los Angeles Lakers and Minnesota Timberwolves are in London preparing for their pre-season opener on Monday at the O2 Arena, while the New York Knicks play Olimpia Milano in Milan on Sunday.
"The NBA is staying in contact with the US embassy, the CIA and Scotland Yard,'' Lakers spokesman John Black said. "They are keeping us informed of the situation.''
Last week British and US media reported that Western spy agencies had foiled an advanced al-Qaeda plan to attack European cities.
The New York Times had earlier quoted an American official as saying the US state department planned to issue the alert on Sunday.
"We have been and continue to be focused on al-Qaeda's interest in attacking us, our allies, and US interests abroad. We will spare no effort to thwart terrorist plans and will take further actions as appropriate," the official said, without elaborating.
Germany said it knew of information pointing to possible al-Qaeda attacks in Europe and the United States.
The plot involving al-Qaeda and allied groups would have involved groups of assailants taking and killing hostages, possibly along the lines of the 2008 raid in the Indian city of Mumbai in which 166 people died, the sources said.
Intelligence sources have claimed that an increase in strikes by unmanned US drone aircraft on suspected fighters in Pakistan in the past few weeks was part of efforts to thwart the suspected plot.
Frequent attacks by US drone aircraft have been heavily criticised by Pakistani leaders for killing innocent bystanders and infringing upon national sovereignty.
Pakistan publicly condemns the attacks as violations of its sovereignty, and the attacks are unpopular among the Pakistani people.
But the Pakistani government is believed to assist in at least some of the missile attacks, although the US does not publicly acknowledge the existence of the covert, CIA-run operation.
The UN has called for the US to end the CIA's control over drone attacks, with Philip Alston, the special rapporteur on extrajudicial executions, saying that the US spy agency does not provide enough transparency or accountability for the programme.
Alston said that the use of drone attacks risked creating a "Playstation" mentality towards killing, "because operators are based thousands of miles away from the battlefield, and undertake operations entirely through computer screens and remote audio-feed".