The United States is sending a guided missile destroyer to the Black Sea in order to reassure European allies in the region following Russia's annexation of Crimea, sources confirmed on Monday.

"We have decided to send a ship into the Black Sea. We expect it will arrive there within a week," said Pentagon spokesman colonel Steven Warren, without disclosing the name or type of the vessel citing "operational security."

A Department of Defense official confirmed to AFP the warship being sent to the region was the USS Donald Cook, a guided missile destroyer.

The boat was recently upgraded to make it capable of firing SM-3 missiles, allowing the ship to function as part of the Aegis Ballistic Missile Defense System.

The boat has been deployed at the Spanish naval base in Rota in order to serve as part of the proposed NATO missile shield.

NATO insists the missile shield is a purely defensive system designed to counter potential missile threats from nations such as Iran. However it has long been a source of tension between NATO and Russia, which sees the project as a threat to its own security.

Warren insisted the deployment of the Donald Cook was meant only to reassure regional allies.

"The purpose is primarily to reassure our allies and partners in the region that we're committed to the region," he said.

"We're still planning the details of our operations in the Black Sea but we expect port calls and exercises with other Black Sea nations."

None of the planned exercises would take place in Ukrainian ports, according to Warren.

Another American destroyer, USS Truxtun, was sent to the Black Sea when the crisis between Russia and Ukraine erupted but left the region on March 21.