WASHINGTON, August 5 (RIA Novosti) - The Pentagon is concerned by the unexpected presence of two Russian submarines in international waters off the U.S. coast, although a Russian official said the patrol was not unusual.

The NewYork Times reported late on Tuesday that the two submarines had been in the area for several days. The newspaper quoted Defense Department officials as saying one of the Akula-class vessels remained 320 kilometers (200 miles) off the coast, while the location of the second was not clear.

One of the submarines is an Akula II nuclear-powered attack submarine, considered the quietest and deadliest of all Russian nuclear-powered attack submarines.

A Pentagon spokesman acknowledged the report. "We have no comment about this. But we are aware of what is happening," he told RIA Novosti by telephone.

Defense Department officials expressed concern to the New York Times about the presence of the vessels due to the unexpected nature of their appearance, but were not worried about the U.S. military's ability to track them.

A U.S. military expert told the paper that it was probably 15 years since two submarines had carried out such a patrol.

That assessment was rejected by a high-ranking Russian Navy source, who told RIA Novosti that the submarines' presence near U.S. waters was not unusual as Russian vessels had never stopped patrolling the world's ocean.

"Even during the fleet's most difficult times in the mid-1990s, Russian submarines put to sea on active alert for patrols. This practice continues to this day," the official said.

The Russian Navy has been more active in recent years, increasing its presence on the world's oceans and carrying out exercises with foreign navies, including in Venezuela last December.

In mid-July the Russian Navy launched two missiles from submarines in the Arctic Ocean. The test was hailed by an intelligence source as a success as the United States had not known about the location of the submarines prior to the launch.

The source said that the launch area, covered by ice floe, was heavily patrolled by Russian attack submarines and the Americans were unable to detect the arrival of two strategic submarines before the launch.