It is US military vessels and not Russia's airplanes that pose a problem in the Black Sea, Russia's Ambassador to Bulgaria Yury Isakov said on Friday.

Speaking for Bulgaria's Darik Radio, His Excellency described the US naval presence in Black Sea waters as a violation of international law, citing the Montreux convention limiting the stay of warships that belong to non-Black Sea countries.

He refuted claims that Russian airplanes were conducting emergency flights near Bulgarian airspace. His Excellency asserted that Russia abided by its international commitments in all of its actions.

"We should rather shift our attention to the increased activity of our American partners," he said, referring to US destroyers USS Truxtun and Donald Cook, which are currently in the Black Sea.

In his opinion, their actions "contravene the Montreux Convention which sets a specifi regime [for warships]," Mr Ambassador Isakov added.

The 1936 Montreux Convention does set a time limit to non-Black sea warships, which are allowed to stay up to 21 days within the sea's waters.

USS Truxtun, the first US destroyer to enter the Black Sea last month for joint naval drills of NATO partners Bulgaria, Romania and the US, has already spent roughly 36 days in the sea since it crossed its Straits on March 6.

It was joined on Friday by another warship, USS Donald Cook, which is reported to be accompanied by a French intelligence surveillance ship, Dupuy de Lome.