WASHINGTON, April 14 -- A Russian fighter jet flew near a U.S. warship in the Black Sea for 12 times on Saturday amid rising tensions in eastern Ukraine, the Pentagon said on Monday.

The USS Donald Cook, a Navy destroyer that arrived in the Black Sea on April 10, was patrolling in international waters in the western Black Sea when an unarmed Russian Su-24 Fencer attack aircraft repeatedly flew nearby, Colonel Steve Warren, a Pentagon spokesman, told reporters.

"The aircraft did not respond to multiple queries and warnings from Donald Cook, and the event ended without incident after approximately 90 minutes," Warren said.

He said two Russian aircraft were present then, but only one was involved and made a total of 12 passes ranging from near sea level to a couple of thousand feet.

The Russian fighter once flew within 1,000 yards (914 meters) of the U.S. ship, an anonymous U.S. military official told reporters. But Warren said the ship was never in danger, noting " The Donald Cook is more than capable of defending itself against two Su-24s."

"This provocative and unprofessional Russian action is inconsistent with international protocols and previous agreements on the professional interaction between our militaries," Warren added.

The chapter was disclosed as tensions were running high in eastern Ukraine, where armed militants were refusing to heed an ultimatum issued by the Ukrainian government to leave official buildings they took over over the weekend.

Washington and its allies accused Russia of being behind the moves, an allegation denied by Moscow.