WASHINGTON - Top lawmakers from the US House of Representatives' intelligence and homeland security panels on Sunday warned of a serious threat of attacks in Russia during the Winter Olympics, though US 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 Feb. 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 US intelligence authorities.



"We're quite satisfied with the level of cooperation we have now," the US Ambassador to Russia, Michael McFaul, said on NBC's "Meet the Press."



Janet Napolitano, head of the US Olympic delegation to Sochi and a former Homeland Security chief, also described the level of security at the Games as very good.



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