WASHINGTON -- Two nuclear-powered Russian attack submarines have been patrolling in international waters off the East Coast for several days, in activity reminiscent of the Cold War, defense officials said Tuesday.

U.S. Northern Command would not comment on the Russian submarines' movement. But in a prepared statement, Northern Command spokesman Michael Kucharek acknowledged the patrols and said the U.S. has been monitoring the two submarines.

Two senior U.S. officials, however, said the submarines had been patrolling several hundred miles off the coast and so far had done nothing to provoke U.S. military concerns. The officials provided details on condition of anonymity in order to discuss intelligence reports.

While the incident raises eyebrows, it did not trigger the more intense reaction by the U.S. military that Russia prompted when two of its bombers buzzed an American aircraft carrier in the western Pacific in February 2008. U.S. fighter planes intercepted the two Russian fighters, including one that flew directly over the USS Nimitz twice at an altitude of about 2,000 feet.

The event did not escalate beyond that, but it signaled a more aggressive military agenda by Moscow.

The latest incident, which was first reported by The New York Times, comes amid increased Russian military activity in the region, and as the administration of President Obama works to thaw tense relations with Moscow over plans for a missile defense system in Central Europe.

Just last week a senior Pentagon official said the administration is looking at options for the plan, which would install 10 interceptors in Poland and a radar system in the Czech Republic. Assistant Secretary of Defense Alexander Vershbow told Congress members that the Obama administration is looking at various configurations as part of its review of missile defense plans.

Russia, meanwhile, conducted naval exercises with Venezuela last year in the Caribbean and sent one of its warships through the Panama Canal for the first time since World War II. The exercises with Venezuela were the first deployment of Russian ships to the Western Hemisphere since the Cold War.

Officials said they became aware of the most recent submarine activity off the East Coast early on through intelligence sources and were not notified by Moscow in advance of the patrols. They said the submarines have not crossed into U.S. waters, which extend 12 miles out into the ocean.

The statement issued by Northern Command and the North American Aerospace Defense Command said, "We have been monitoring them during transit and recognize the right of all nations to exercise freedom of navigation in international waters according to international law."