Cyber attacks could be launched on "any company that finances or supports" the Sochi Olympics, U.S. security officials warned Wednesday as hackers targeted Internet sites related to the Russian games.
A security bulletin issued by the Homeland Security Department's U.S. Computer Emergency Readiness Team warned that the winter games could be used by "hacktivists" seeking to "take advantage of the large audience to spread their own message."
It cites one such group, Anonymous Caucasus, which claims "the Sochi games infrastructure was built on the graves of 1 million innocent Caucasians who were murdered by the Russians in 1864," according to the Homeland Security bulletin.
Anyone attending the games "will likely" have their communications monitored, the bulletin warned, and cyber criminals may use the games as a way to spread malware or phishing scams to sports audiences around the world.
The alert came as a denial of service attack was launched on Internet sites related to the games, knocking Russia's national games committee offline Wednesday.
Anonymous Caucasus announced on Twitter that it had called on its members to take action against Sochi-related sites, using the hashtags #paybackforsochi and #opsochi.
The official web site for the Russian National Olympic Committee, Olympic.ru, was unavailable for a time and appeared to have been targeted in the action.
Tweets and online postings from some hackers said the action was in protest of the reported killing of stray dogs at Olympic venues earlier this week as media and athletes arrived for the games, which begin on Friday.
A list of 700 Russian targeted websites was headed with the message: "We warned you Russia."
On a related Twitter account, hackers posted a message to the Russian government and media: "Your cruelty to animals is enough to take down Your entire country, and we plan on it. Welcome to WAR Russia."
However, the official Twitter feed of Anonymous Caucasus said the action was a protest at the 19th Century deportation of thousands of native Circassians from the region.
(Read more: Welcome to Sochi: You're hacked!)
Troops from Tsarist Russia forcibly removed the ethnic group, and many perished. Descendants are campaigning to have the episode recognized as genocide.
Earlier, Anonymous claimed to have attacked the site sochi-airport.com and a website of the Olympic sponsor Sberbank, the largest bank in Russia. Both sites were unavailable at times.
The hackers also targeted the site 2014.tpu.ru - an online training center set up for Olympic volunteers. It was unavailable for a period of time on Wednesday.
Anonymous Caucasus asked members and supporters to attack Bosco Sports, the supplier of official apparel for Olympic staff and volunteers, including the runners in the torch relay.
By NBC News's Carlo Angerer and Michael Isikoff