* Risks include polluting intelligence streams
* Plot involves attacks on European targets
* Top al Qaeda leadership believed involved
By Phil Stewart
WASHINGTON, Oct 4 (Reuters) - The U.S. warning to citizens of an increased risk of terrorist attacks in Europe also sent a clear message to militants: "We're on to you."
The repercussions could cut both ways, U.S. officials acknowledged on Monday.
"There is a significant impact on counterterrorism work when the enemy realizes you are on to their game," one U.S. official said, speaking on condition of anonymity.
Known suspects could stop communicating, go underground, or try to weed out spies. A publicly acknowledged threat can also add to confusion for Western intelligence officials, deluging them with questionable tips, misinformation and false alarms.
But the alert could also disrupt plans and prompt militants to re-think an attack.
That makes decisions like Sunday's State Department broadcast of a threat to the general public more complicated.
The trigger for the travel alert was intelligence about a plot against European targets originating in Pakistan's tribal areas. Senior al Qaeda leaders, including Osama bin Laden, are likely involved, U.S. officials say.
The advisory itself was extraordinarily vague, blanketing all of Europe under a generic warning that essentially told U.S. citizens to be on alert. It said public transportation systems and other tourism-related facilities could be targets.
It did not warn citizens against going to Europe, but instead to take precautions if they did.
"As the general nature of the alert suggests, the precise timing and target of potential terrorist operations aren't known," a different U.S. official said.
Paul Pillar, a former CIA counterterrorism official now with Georgetown University, said that so little was revealed in the advisory that fallout was unlikely.
"When what is made public is as general and non-specific as is the case with this advisory about Europe, it's hard to imagine that any specific sources and methods are jeopardized," Pillar said.
At the same time, he added that would-be attackers read newspapers too and that media reports offering more details could tip off militants as well.
"There are opportunities as well as costs," Pillar said. (Editing by Doina Chiacu)
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