A Russian intelligence-gathering ship has been operating off the U.S. East Coast and near the Gulf of Mexico for the past month, the Pentagon said Thursday.

"We are aware that the Russian ships Viktor Leonov and Nikolay Chiker are currently operating in waters that are beyond U.S. territorial seas but near Cuba," said Lt. Col. Tom Crosson, a Pentagon spokesman. "We respect the freedom of all nations, as reflected in international law, to operate military vessels beyond the territorial seas of other nations."

The Leonov is an intelligence gathering ship outfitted with high-tech electronic spying gear. The Chiker is an ocean-going naval tug that has been accompanying the spy ship on its mission.

Pentagon officials suspect the ships were part of a spying operation since March against the U.S. nuclear missile submarine base at Kings Bay, Ga. and other U.S. military facilities

Both ships were detected operating off the coast of Florida near the U.S. Naval Station Mayport, Fla., which is south of the Naval Submarine Base Kings Bay.

The Russian intelligence gathering coincides with heightened tensions between the United States and Russia over Moscow’s recent military annexation of Ukraine’s Crimea.

An official said it is possible that the electronic spying is related to watching U.S. nuclear missile submarines as part of a Russian nuclear exercise.

According to Russian military press reports, some 10,000 Russian troops and 1,000 pieces of military equipment of the Strategic Missile Forces took part in an exercise April 17 to 19—coinciding with the transit of one of the ships, the Chiker, to Cuba from the coast off northern Florida on April 19.

"The exercises will test the cohesiveness and skills of units and commands in the process of alerting and the achievement of training objectives under various circumstances and in any time of the day," Russian defense spokesman told Interfax.

The Chiker also is known to support submarines and is equipped with lift capability for servicing Russian submarines.

U.S. officials said in 2012 a Russian Akula attack submarine was detected operating near the East Coast. The Navy denied the sub sailed undetected in the Gulf of Mexico.

Crosson declined to comment further on the Russian naval activities. "It wouldn't be appropriate for me to talk about the operations of non-U.S. vessels operating beyond U.S. territorial seas," he said.

According to military enthusiast websites, the Leonov is a Vishnya-class medium intelligence ship home ported in Severomorsk and is part of the Kola Peninsula naval forces.

The ship was commissioned in 1988.

The ships are designed for signals and communications intelligence gathering through an array of ship-borne sensors. It also is equipped with two 30-millimeter guns and anti-aircraft missiles.

Wire services reports indicated the Leonov docked in Havana in February and March, and again this month.