* U.S. says al Qaeda and affiliated groups prepare attacks
* Britain increases threat level for France and Germany
* Plot could involve European citizens or residents (Updates with US comment on touring, graph 12, French official graphs 16, 17)
By Caren Bohan and Kylie MacLellan
WASHINGTON/LONDON, Oct 3 (Reuters) - The United States and Britain warned their citizens on Sunday of an increased risk of terrorist attacks in Europe, with Washington saying al Qaeda might target transport infrastructure.
The U.S. State Department issued a warning directed at American citizens traveling in Europe, without singling out any specific countries, saying tourists should proceed with caution.
Britain raised the terrorism threat level in its advice for citizens traveling to Germany and France to "high" from "general." It left the threat level at home unchanged at "severe," meaning an attack is highly likely, and said it agreed with the U.S. assessment for the continent as a whole.
The moves came after a week in which a number of European officials had broadly confirmed media reports that new intelligence indicated possible attacks on the continent.
Western intelligence sources said militants in hide-outs in northwest Pakistan had been plotting coordinated attacks on European cities, the plans apparently surviving setbacks from a September surge in drone strikes and an arrest.
U.S. warning of possible attacks in Europe [ID:nN03245464]
UK says Europe terror threat "high" [ID:nLDE6920B7]
FACTBOX-Suspected groups [ID:nLDE6920CO]
FACTBOX-Islamist militant attacks in Europe[ID:nLDE6920CK]
Alert has no initial impact on U.S airlines[ID:nN03264427]
The plot involved al Qaeda and allied militants, possibly including European citizens or residents, the sources said. In Washington, U.S. officials said Osama bin Laden and the top al Qaeda leadership were likely behind the plot, adding that the decision to issue the alert was based on an accumulation of information, rather than a specific new revelation.
Some security officials have drawn comparisons to the brazen Mumbai attacks in 2008 that targeted city landmarks such as luxury hotels and a cafe and killed 166 people.
The U.S. State Department travel alert said public transportation systems and other tourism-related facilities could be targets, noting that past attacks had struck rail, airline and boat services.
"The State Department alerts U.S. citizens to the potential for terrorist attacks in Europe," it said in an advisory on its website.
A U.S. official said President Barack Obama held meetings on Friday night and Saturday morning about the European security threat and was briefed on the situation again on Sunday morning.
The alert was posted on the State Department website at http:/travel.state.gov/travel/cis_pa_tw/pa/pa_europe.html.
Europe is worried about how reports of the threats might affect tourism. The U.S. alert falls short of a more severe one in which the State Department might have warned citizens against traveling to Europe. Instead, the alert urges them to take precautions when they do travel.
"We're not saying don't travel to Europe. We are not saying don't visit ... major tourist attractions or historic sites or monuments," Patrick Kennedy, undersecretary of state, told reporters on a conference call.
Said U.S. tourist Tom Steier: "You should take these threats seriously, but right now I feel very safe in Paris."
French Foreign Ministry spokesman Bernard Valero said France had taken the U.S. warning into account: "The terrorist threat remains high in France ... the alert level remains unchanged at red," he said. That is the second highest level.
While anti-terror measures have been mobilized, French Interior Minister Brice Hortefeux told reporters late on Sunday that "We shouldn't be in denial."
"There are several bits of information that have come to light in the last few days, which is what the Americans are confirming today," he said.
A spokesman for Britain's Foreign and Commonwealth Office (FCO) declined to detail why the travel advice for France and Germany had been updated. "Like other large European countries, they have a high threat of terrorism, which is reflected in our updated advice."
Advice is under constant review and draws on a variety of sources, the FCO said.
A new potential threat was outlined last week in media reports, partially confirmed by European officials, that Europe could be the target of attacks.
European Union counter-terrorism coordinator Gilles de Kerchove said the plot showed the continent had to do more to impede extremists going overseas to train.
Sweden raised its terrorism threat level on Friday, although it said it was still lower than in other European countries and an attack was not believed to be imminent.
The last successful large-scale militant attacks in Europe were the July 2005 suicide bombings on London's transport system, which killed 52 people. Bombers killed 191 people on trains in Madrid in March 2004. (Writing by Peter Graff; Editing by Jon Boyle and Philip Barbara)
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