Washington: The US State Department issued a formal alert warning Americans travelling in Europe to remain vigilant against "potential for terrorist attacks" and urging precaution in public places and transportation systems.
France and Britain immediately voiced support for the security statement, which said "current information suggests that al Qaeda and affiliated organisations continue to plan terrorist attacks”.
"US citizens should take every precaution to be aware of their surroundings and to adopt appropriate safety measures to protect themselves when travelling," according to the alert.
It said attackers may use "a variety of means and weapons and target both official and private interests”, and that particular targets could include railways, subways and locations frequented by tourists.
The alert -- which the State Department issues regarding specific events, and is one step down from a travel warning -- follows intelligence reports that suggested an al Qaeda attack could be imminent.
"European governments have taken action to guard against a terrorist attack and some have spoken publicly about the heightened threat conditions," the State Department said.
News media in the past week reported that Western intelligence agencies had uncovered an al Qaeda plot to launch attacks in Britain, France, Germany and the United States.
The reports said well-armed teams of jihadists planned to seize and murder Western hostages in a manner similar to the attacks two years ago in the Indian city of Mumbai on two hotels and its main railway station, in which 10 gunmen killed 166 people and injured more than 300.
US President Barack Obama has been "following the threat information on a daily basis and was informed on the travel alert throughout”, White House spokesman Nicholas Shapiro told reporters.
"From the day we became aware of this latest plot, the President made clear we need to do everything possible to disrupt this plot and protect the American people."
Under Secretary of State for Management Patrick Kennedy, who acknowledged that such a terror alert for Europe was rare, would not go into the intelligence behind the security statement, but told reporters "we`ve been monitoring this (situation) carefully for at least several weeks”.
Minutes after it was issued, Britain and France expressed support, while the European Commission said it was "monitoring the situation”.
In Berlin, the Interior Ministry said on Sunday, however, there was no reason to change Germany`s security threat level in response to a US travel alert warning of potential terrorist attacks in Europe.
British Home Secretary Theresa May said the US alert was "consistent with our assessment”. Britain updated its own travel advice for France and Germany on Sunday, warning there was a "high threat of terrorism”.
The Foreign Ministry in France -- the most visited country in the world, with 74.2 million visitors recorded in 2009 -- said through a spokesman that the US alert is "in line with the general recommendations we ourselves make to the French population”.
In Paris, bomb scares briefly shutting down the Eiffel Tower and train stations in recent weeks.
An Italian Foreign Ministry official said that "the fight against terrorism is one of our priorities and on this subject we are in tune with the United States”.
The German weekly Der Spiegel reported that an al Qaeda plot to launch attacks on European cities was planned by the group`s number three leader, with support from Osama bin Laden.
In its issue to hit newsstands Monday, Der Spiegel reported that al Qaeda`s third in command, Sheikh Yunis al-Mauretani, plotted the attacks, and had shared his plans with Ahmad Siddiqui, an Islamist with German nationality currently held at the US-run Bagram Air Base in Afghanistan.
Siddiqui was the likely source of information that sparked recent hikes in Western security threat levels, the weekly said.
A travel alert could have a substantial effect on economic segments in Europe, which earned about USD 412 billion from tourism in 2009, according to UN World Tourism Organisation figures.
Some 10.6 million Americans reportedly visited Europe last year.
A travel alert currently exists for India through November 15 due to the 2010 Commonwealth Games to be held in New Delhi.
Kennedy said the last travel alert prior to Sunday`s was a worldwide one issued on September 09 urging Americans to "exercise caution due to possible anti-US demonstrations in response to stated plans by a Florida church to burn Qurans”.