MOSCOW -- Russia's Defense Ministry Thursday refused to unveil the patrol routes of its strategic submarines, calling such information "classified."

"Currently, Russian submarines are on combat missions in various areas of the world's oceans," the ministry said in a statement.

The ministry will not disclose the routes, activities or combat tasks of Russia's submarine fleet, in accordance with international practice, the ministry said, adding that such classified information could remain secret for decades.

A U.S. newspaper reported Tuesday that a nuclear-powered Akula- class Russian submarine operated in restricted U.S waters up to a month without being detected.

The Washington Free Beacon, quoting U.S. officials, said the attack warship traveled in the Gulf of Mexico in June and July. The submarine's presence was only confirmed after its departure from the region, said the report.

The Russian Defense Ministry didn't comment directly on Tuesday 's report, saying that foreign media have frequently reported spotting Russian submarines' whereabouts overseas since the fleet and single submarines resumed long-distance patrol "in many parts of the ocean and far-off seas."