WASHINGTON — A pair of nuclear-powered Russian attack submarines has been patrolling off the eastern seaboard of the United States in recent days, a rare mission that has raised concerns inside the Pentagon and intelligence agencies about a more assertive stance by the Russian military.

The episode has echoes of the cold war era, when the United States and the Soviet Union regularly parked submarines off each other’s coasts to steal military secrets, track the movements of their underwater fleets — and be poised for war.

But the collapse of the Soviet Union all but eliminated the ability of the Russian Navy to operate far from home ports, making the current submarine patrols thousands of miles from Russia more surprising for military officials and defense policy experts.

“I don’t think they’ve put two first-line nuclear subs off the U.S. coast in about 15 years,” said Norman Polmar, a naval historian and submarine warfare expert.

The submarines are of the Akula class, a counterpart to the Los Angeles class attack subs of the United States Navy, and not one of the larger submarines that can launch intercontinental nuclear missiles.

According to Defense Department officials, one of the Russian submarines remained in international waters on Tuesday about 200 miles off the coast of the United States. The location of the second remained unclear. One senior official said the second submarine traveled south in recent days toward Cuba, while another senior official with access to reports on the surveillance mission said it had sailed away in a northerly direction.

The Pentagon and intelligence officials spoke anonymously to describe the effort to track the Russian submarines, which has not been publicly announced.

The submarine patrols come as Moscow tries to shake off the embarrassment of the latest failed test of the Bulava missile, a long-range weapon that was test fired from a submarine in the Arctic on July 15. The failed missile test was the sixth since 2005, and some experts see Russia’s assertiveness elsewhere as a gambit by the military to prove its continued relevance.

“It’s the military trying to demonstrate that they are still a player in Russian political and economic matters,” Mr. Polmar said.

One of the submarines is the newer Akula II, officials said, which is quieter than the older variant and the most advanced in the Russian fleet. The Akula is capable of carrying torpedoes for attacking other submarines and surface vessels as well as missiles for striking targets on land and at sea.

Defense Department officials declined to speculate on which weapons might be aboard the two submarines.

While the submarines have not taken any provocative action beyond their presence outside territorial waters of the United States, officials expressed wariness over the Kremlin’s motivation for ordering such an unusual mission.

“Anytime the Russian Navy does something so out of the ordinary it is cause for worry,” said a senior Defense Department official who has been monitoring reports on the submarines’ activities.

The official said the Navy was able to track the submarines as they made their way through international waters off the American coastline. This can be done from aircraft, ships, underwater sensors or other submarines.

“We’ve known where they were, and we’re not concerned about our ability to track the subs,” the official added. “We’re concerned just because they are there.”

Once among the world’s most powerful forces, the Russian Navy now has very few ships regularly deployed on the open seas. Moscow has contributed warships to the international armada searching for Somali pirates.

Another example of how Russia’s navy has sought to display global reach came last year when a flotilla of warships sailed for exercises with Venezuela.

The submarine patrols off the East Coast follow Russia’s resumption last year of bomber runs off the coast of Alaska. Russia began sending Tu-95 Bear bombers through international airspace near Alaska in what was interpreted as a signal of the Kremlin’s unhappiness over decisions by the United States and Europe to recognize Kosovo’s independence, in defiance of Russia.