A Russian intelligence-gathering ship is again plying the waters off the southern United States in operations aimed at spying on U.S. ballistic missile submarines based in the area, defense officials said.

The intelligence collection ship, Viktor Leonov, has been closely watched by U.S. Navy ships and aircraft for the past several days near Jacksonville, Fla., close to the Naval Submarine Base at Kings Bay, Ga. The ship also conducted operations there in April.

The spying comes amid heightened U.S.-Russia tensions over the crisis in Ukraine, where Russian forces annexed the Crimea last year and are continuing to arm pro-Moscow rebels in the eastern part of the country.

The Kings Bay base is homeport for the Navy’s Submarine Group 10, with six nuclear-armed missile submarines and two conventionally armed missile submarines.

"It’s been all in international waters and all perfectly legal," said a defense official familiar with efforts to monitor the ship. "But it’s interesting that it is operating, collecting on us where it is."

This week, the Leonov was spotted anchored about 22 miles off the Florida coast, southeast of Kings Bay.

It reportedly left Cuba on Jan. 22, and its movements since then have not been made public.

The ship, known as an AGI in military parlance, is equipped with high technology gear designed to pick up electronic communications and underwater signals. It is also armed with 30-millimeter cannon and anti-aircraft guns.

The Leonov recently made headlines by making a port visit to Havana, Cuba, in late January that coincided with the Obama administration’s initiative to normalize relations with the communist regime in Cuba.

The spy ship’s presence also comes as Russia is increasing the number of strategic bomber flights near U.S. and allied coasts. One recent air defense zone incursion took place near eastern Canada that U.S. officials said simulated a nuclear cruise missile attack on the United States.

The Tu-95 flights appear to have subsided in recent weeks, defense officials said.

The director of the Defense Intelligence Agency, Marine Corps Lt. Gen. Vincent R. Stewart, told Congress last week that Russia is building up nuclear forces and increasing out-of-area operations, including in the Caribbean.

"Russian forces have conducted exercises and … record numbers of out-of-area air and naval operations," Stewart said in testimony to the House Armed Services Committee.

" We expect this to continue this year to include greater activity in the Caribbean and Mediterranean Seas."

On Moscow’s nuclear buildup, Stewart said: "Moscow has made significant progress in modernizing its nuclear and conventional forces, improving its training and joint operational proficiency, modernizing its military doctrine to integrate new methods of warfare, and developing long range precision strike capabilities."

"We anticipate continued high levels of Russian military activity in 2015," he added.

New Russian strategic forces include additional deployments of an advanced version of the road-mobile SS-27 missile with multiple warheads, the new RS-26 missile, and the SS-N-32 submarine launched ballistic missile.

In April, the Washington Free Beacon revealed that the Leonov, along with the Nikolay Chiker, a naval tug capable of serving Russian submarines, were conducting operations off the East Coast.

The Pentagon played down the vessels’ presence as outside U.S. territorial waters, but near Cuba.

Russia, under President Vladimir Putin, has shown a revival of Cold War rhetoric and nuclear activities, including large-scale strategic nuclear exercises and a build up of nuclear forces.

In 2012, the Navy detected a Russian Akula attack submarine near the East Coast. In a conflict, Akula submarines would be tasked to find and sink U.S. missile submarines.