Top lawmakers from the U.S. House of Representatives' intelligence and homeland security panels on Sunday warned of a serious threat of attacks in Russia during the Winter Olympics, though U.S. officials say the Olympic grounds are secure.
The Olympics formally opened on Friday in Sochi. Islamist militant groups based in the nearby north Caucasus region have threatened attacks during the February 7-23 Games.
"I've never seen a greater threat certainly in my lifetime," said House Homeland Security Committee Chairman Michael McCaul, a Republican from Texas.
"I think there's a high degree of probability that something will detonate, something will go off, but I do think it's probably most likely to happen outside of the ring of steel and the Olympic Village," he said on "Fox News Sunday".
Some 37,000 security personnel are on high alert in Sochi and U.S. officials on Sunday said cooperation has improved, though still not enough, between Russian and U.S. intelligence authorities.
"We're quite satisfied with the level of cooperation we have now," the U.S. Ambassador to Russia, Michael McFaul, said on NBC's "Meet the Press."
Janet Napolitano, head of the U.S. Olympic delegation to Sochi and a former Homeland Security chief, also described the level of security at the Games as very good.
"Within the boundaries of Sochi, within the so-called ring of steel, there's a lot of security," she said on CNN's "State of the Union" on Sunday. "I hope that the attention of the media and the world turns now more to what the athletes are going to do instead of the threats that are being made."
Separatist guerrillas seeking an independent Islamic state in Chechnya and neighboring regions of southern Russia have vowed to disrupt the Olympics, which they say are taking place on land seized from Caucasus tribes in the 19th century.
"The guards, gates and guns portion of this is really unparalleled for an Olympic game," said House Intelligence Committee Chairman Mike Rogers, a Republican from Michigan, adding that "internationally, intelligence is as good as I've seen it."
Despite a "ring of steel" around venues, Russian security forces were last month hunting a woman suspected of planning a suicide bombing and who may already be in Sochi. Rogers, speaking on ABC's "This Week" on Sunday, said the search was continuing.
One issue is sharing of intelligence between Washington and Moscow.
"There has been some more sharing than there had been, still not what it should be," Representative Peter King, a New York Republican who sits on the House Homeland Security and Intelligence committees, said on CBS's "Face the Nation".
"(Russians) are still reluctant to give intelligence that they feel would allow us to determine their sources and methods. And also, there's a certain amount of pride. I believe that they feel they can handle a lot of this on their own."
Security analysts believe an attack is more likely elsewhere in Russia to humiliate President Vladimir Putin, who launched a war to crush a Chechen separatist rebellion in 1999.
Putin has staked a great deal of personal and political prestige on the Games and hopes they will showcase Russia's modern face to the world and help its weakening economy.
The United States on Friday issued a fresh travel alert for Americans attending the Sochi Olympics, citing cyber security threats and warning them to have "no expectation of privacy" using Russian communications networks.
The U.S. State Department's alert - coming the same day that Turkish security forces in Istanbul seized a Ukrainian man accused of trying to hijack an airliner and redirect it to Sochi - updated one issued two weeks earlier.
The U.S. Transportation Security Administration temporarily banned carry-on liquids, aerosols, gels and powders on flights between Russia and the United States.