Defense Secretary Chuck Hagel visited this naval cruiser in the Black Sea on Thursday to reinforce the U.S. commitment to protect NATO allies in Eastern Europe anxious about Russian moves in nearby Ukraine.

The ship, equipped with an Aegis missile defense system, is part of a continuous U.S. naval presence here since mid-March. “We will sustain this tempo going forward,” Hagel said.

In the Romanian port city of Constanta, Hagel told Defense Minister Mircea Dusa that the $1 billion that President Obama said this week he plans to request from Congress would help pay for “a stronger presence of U.S. ships in the Black Sea” as well as “more U.S. troop rotations for exercises and training” with allied states in the region.

Hagel’s trip to Romania, between his attendance at a meeting of NATO defense ministers in Brussels that was dominated by the Ukraine crisis and a rendezvous with Obama to attend the 70th anniversary of the D-Day landings in Normandy, is part of the administration’s effort to bolster NATO’s defenses on its eastern flank as a warning to Russia.

Obama delivered the same message earlier in the week in Poland, where the United States has sent additional F-16 aircraft. Vice President Biden visited Romania less than a month ago.

In the decade since it joined NATO, Romania has been among the most ardent members of the alliance, sending troops to Iraq and Afghanistan. Dusa said Wednesday that his government would increase its defense budget to 2 percent of the gross national product by 2016, making it one of the few NATO governments meeting alliance spending targets.

Three months ago, Romania’s MK base became the main air transit point for U.S. troops and cargo entering and leaving Afghanistan, after Kyrgyzstan refused this year to renew a U.S. lease at an air base there. More than 50,000 troops transited the Romanian base in its first 100 days of operation.

Romania, along with Poland, is the site of ground-based elements of the missile defense system that the United States is installing in Europe. The Romanian site, part of the second phase of the system, is scheduled for completion next year.

The Vella Gulf is the fourth U.S. ship sent to the Black Sea since a maritime rotation began in March. “The idea is . . . not [to be] steaming up and down the beach of Crimea,” the autonomous Ukrainian peninsula annexed by Russia in March, but to be “a visible presence in the Black Sea,” a senior defense official traveling with Hagel said.

Under international agreements administered by Turkey, naval vessels not belonging to Black Sea states can stay in the waters for only 21 days. The Vella Gulf arrived May 23 and will be replaced by another ship when it departs, the defense official said.

“We’re grateful for what you’re doing at a very important time in the world, in particular this mission that you’re on,” Hagel told sailors aboard the ship in a message broadcast over the intercom system.