In the latest sign of support for NATO allies in the wake of Russia's annexation of Crimea, the U.S. Navy guided missile destroyer USS Donald Cook will enter the Black Sea on Thursday to participate in exercises with regional partners. It is unclear how long the destroyer will remain in the Black Sea.

Pentagon spokesman Col. Steve Warren announced Wednesday that the destroyer would enter the Black Sea on Thursday for unspecified exercises with partner nations in the Black Sea region. He said the ship's presence in the Black Sea "demonstrates our commitment to our allies and to enhance security readiness in the region."

A statement from U.S. Naval Forces Europe-Africa said "Donald Cook's mission is to reassure NATO allies of the U.S. Navy's commitment to strengthen and improve interoperability while working toward mutual goals in the region."

Ever since Russian troops first appeared in Crimea in February, the United States has sought to reassure NATO allies by raising its military profile in Eastern Europe .

In early March, Defense Secretary Chuck Hagel ordered aviation units to Poland and Lithuania to bolster existing operations, with 12 F-16s and 200 airmen sent to Poland on an open-ended training schedule, augmenting a 10-man aviation detachment based in that country for rotating U.S. exercises. An additional six F-15 fighters were also sent to Lithuania to augment the four U.S. F-15s participating in NATO's Baltic Air Policing Mission.

The likelihood that another U.S. Navy ship was headed to the Black Sea emerged last week, but Defense officials were wary of identifying the ship because of potential schedule conflicts. The destroyer USS Truxtun was in the Black Sea for two weeks in March conducting exercises with Romanian and Bulgarian navy ships. Although the Navy says it is routine for its ships to enter the Black Sea, those visits do not typically take place weeks apart.

Russia's massing of 40,000 troops along its border with eastern Ukraine for what it calls military exercises spurred words of caution from senior U.S. and NATO officials concerned that the troops might push into eastern Ukraine.

Amid those concerns, NATO Supreme Allied Commander Gen. Philip Breedlove was asked by NATO to develop additional military options to reassure the alliance's eastern European members. He is expected to present those options to the NATO Council on April 15.

For now, the exercises the Donald Cook will be participating in have yet to be defined and it remains unclear how long the ship will remain in the Black Sea. The destroyer is the first of four Arleigh Burke-class destroyers to have been deployed to its new home port of Rota, Spain.

Also on Thursday, six F-16s and 180 airmen based at Aviano, Italy will arrive in Romania for a week-long exercise with the Romanian air force that was planned months before the current crisis in Ukraine.