HAWAII -- The U.S. Coast Guard announced Thursday that it is tracking a suspected Russian ship sailing off the coast of Hawaii in recent weeks.
The Pentagon said the ship is in international waters and behaving professionally.
But what could it be looking for?
"I can't speak to why the Russians are sailing the ship right now, it's kind of precarious timing," said Pentagon spokeswoman Sabrina Singh.
The Coast Guard believes it may not just be any ship, but instead, a vessel the coast guard said "is believed to be an intelligence gathering ship." In other words, a spy ship.
While the appearance of a Russian surveillance ship along the U.S. coastline is not unusual, this one has attracted more attention because of heightened tensions between the U.S. and Russia over Moscow's invasion of Ukraine and veiled threats to employ nuclear weapons.
That is why the U.S. Coast Guard is carefully monitoring the Vishnya-class Kareliya surveillance ship that has been seen prowling the waters off of the Hawaiian Islands.
"Ships loitering in an area is abnormal," said ABC News contributor and retired Navy commander Eric Oehlerich. "And with this one being a Russian vessel off one of our key naval harbors in the Pacific -- that will be of concern to the naval leadership."
Indeed, joint base Pearl Harbor is one of the Navy's busiest, with some of the most sophisticated and secretive warships in its arsenal. It's also where ABC News boarded a boat that took us to a nuclear armed ballistic missile submarine last year off Hawaii's coast.
"The submarines do not like to stay above water for long because then they are detectable," ABC News Martha Raddatz said in a previous report.
It is exactly the kind of submarine the Russians could try to learn more about by dropping a tracking device underwater.
But there is another possibility -- sabotage.
"They could be mapping some of our critical infrastructure that comes in and out of Hawaii. Fiber optic internet and communication cables, for example, would be one of those," Oehlerich said.
"We haven't seen any unsafe or unprofessional behavior and we expect that the Russians will operate within the region in accordance with international law," Singh said.
In a video posted to Twitter by the Coast Guard, the ship appears to be either being towed by another ship or in a resupply mode.
"U.S. Indo-Pacific Command is monitoring the Russian vessels operating in international waters in the Western Pacific," the Pentagon said in a statement. "As Russia operates within the region, it is expected to do so in accordance with international law."
The Associated Press contributed to this post.