The USS Destroyer Donald Cook is seen at the Constanta shipyard in the Romanian Black Sea port of Constanta, on April 14, 2014.
A Russian fighter jet made several passes at low altitude near a US destroyer cruising in international waters in the Black Sea at the weekend, the Pentagon said on Monday, branding it "provocative and unprofessional."
The incident close to the Romanian coast further heightens tensions already inflamed by Russia's actions in Ukraine, where Moscow annexed the Crimean peninsula and stands accused of stoking unrest.
The US vessel was sent to the Black Sea on April 10 in a show of Washington's solidarity with its Eastern European NATO allies concerned about Russia's actions.
Colonel Steven Warren, a Pentagon spokesman, said: "On April 12, a Russian Su-24 made numerous close-range, low-altitude passes in the vicinity of the USS Donald Cook, while the Cook was conducting operations in international waters in the western Black Sea.
"The aircraft did not respond to multiple queries and warnings from Donald Cook. The event ended without incident after approximately 90 minutes."
He added: "This provocative and unprofessional Russian action is inconsistent with international protocols and previous agreements on a professional interaction between our militaries."
During some passes, the Russian plane came close to less than 1,000 meters (3,300 feet) from the American ship, a military official told AFP on condition of anonymity.
The plane "appeared to be unarmed," Warren said, adding that it fluctuated between flying up to several thousand feet in the air and almost skimming the water.
"It did not overfly the deck of the Donald Cook."
A second Su-24 was nearby, but was not involved, Warren said, warning: "The Donald Cook is more than capable to defend herself against two Su-24s."
The US vessel was on a routine patrol, he said, and "was never under threat. This was simply a provocative act."
Warren added: "I have difficulty believing that two Russian pilots on their own would choose to take such an action. We've seen the Russians conduct themselves unprofessionally and in violation of international norms in Ukraine now for several months."
The USS Donald Cook has no aircraft but carries an array of armaments including torpedoes and Tomahawk cruise missiles.