NATO allies Bulgaria and the United States will stage a series of joint military drills over the next three and a half months, the Bulgarian defence ministry said on Friday, amid increased tensions with Russia over the Ukraine crisis.

Some 350 U.S. Army officers will arrive in the Balkan country to take part in the bilateral drills, which begin on Sunday and will be conducted at the Novo Selo training range in southeastern Bulgaria.

The exercises will also involve U.S. armoured personnel carriers, helicopters and tanks.

"The Bulgarian Army will use the drills to test its ability to react in crisis situations," a defence ministry official told Reuters. "Of course, we cannot underestimate what is happening in Ukraine."

Defence Minister Nikolay Nenchev said the United States would spend about $30 million to modernise the infrastructure at and around the training range of Novo Selo.

Bulgaria and other ex-communist countries in eastern Europe that are now inside NATO and the European Union have been rattled by Russia's annexation of Ukraine's Black Sea peninsula of Crimea and its support for separatists in eastern Ukraine.

On Tuesday, NATO's Black Sea members Bulgaria, Romania and Turkey joined four other alliance states - the United States, Canada, Germany and Italy - in a multi-national naval exercise in the Black Sea.

Bulgaria, a close ally of Moscow in communist times, joined NATO in 2004 and is one of six eastern European countries due to host new command units staffed with national and NATO soldiers being set up in response to the Ukraine crisis.