NATO ships from six countries have commenced exercises off the coast of Bulgaria in the Black Sea, 500 kilometers (310 miles) from Russian-held Crimea. The training, which started Tuesday and will last a little over two weeks, comes as a Russian officials say they have the right to position nuclear weapons in the peninsula that was annexed from Ukraine last March. 

"I don't know if there are nuclear weapons there now. I don't know about any plans, but in principle Russia can do it," said Mikhail Ulyanov, head of the Russian Foreign ministry's department on arms control, according to the Interfax news agency.

Ships from the United States, Canada, Italy, Romania, Turkey and Bulgaria are taking part in the maneuvers near the Bulgarian port of Varna. Exercises will include anti-aircraft and anti-submarine warfare as well as general preparedness training.

U.S. Rear Adm. Brad Williamson will lead the operation onboard U.S. guided missile cruiser USS Vicksburg.

“The training and exercises we will conduct with our allies in the Black Sea prepares us to undertake any mission NATO might require to meet its obligations for collective defense,” said Williamson on the alliance’s Maritime Command website.

This particular exercise comes at a time of heightened tensions between the alliance and Russia, which NATO accuses of  assisting pro-Russian forces in the eastern Ukraine war since it began in April. While a ceasefire has been in place for almost one month since coming into force on Feb. 15, fighting between the Ukrainian military and pro-Russian forces has hardly stopped. However, news that rebels were withdrawing heavy weapons from the front has given fresh hope that the ceasefire eventually may stick.

In the face of accusations that the NATO Black Sea exercises are hostile toward Russia, NATO countered that alliance ships regularly visit the Black Sea for training.

"NATO regularly deploys ships to the Black Sea for maritime awareness and training. This scheduled deployment, given Russia's continued assertiveness, carries an additional message of reassurance to allies in the region," a NATO official told Reuters.

However, Russia’s envoy to NATO, Aleksandr Grushko, previously has said that Moscow would take “necessary countermeasures” if the alliance continued its military buildup in the region.

Since the annexation of Crimea and the war in east Ukraine, NATO has increased its activities significantly in Eastern Europe, centered around a deployment of troops and equipment known as Operation Atlantic Resolve, across the Baltic states and down to the Black Sea.

Russian Deputy Defense Minister Anatoly Antonov accused NATO of using the conflict in east Ukraine to close in on Russia’s border, but said that Russia did not see the Black Sea exercises and Operation Atlantic Resolve as an immediate threat.  

"Instead of uniting forces to fight evil, the worst of which is terrorism, Western nations are drawing new divisive lines, trying to realize containment schemes against unwelcome states. Today, Russia has been chosen as the target,” Antonov said on March 5 in a Russian TV broadcast.

However, access to the Black Sea for warships belonging to nations not on the Black Sea itself, which includes most of NATO, is limited by the 1936 Montreux Convention. The treaty states that warships of non-Black Sea powers can only remain in the sea for 21 days and must be under 15,000 tons, so the U.S. and NATO allies excluding Bulgaria, Romania and Turkey have to circulate ships in and out and cannot send in large destroyers or aircraft carriers. The Vicksburg weighs just under 10,000 tons.  

Russia is legally allowed to have any number of ships and aircraft carriers in the water because of its Black Sea coastline, giving it a major advantage in the event of a war. The Black Sea NATO members do not have aircraft carriers, which means NATO is excluded from having such an asset in the region.