"Bulgaria and the Allies continue regular enhanced surveillance activities in the NATO area of responsibility," the defence ministry said, explaining that there was no cause for concern as the exercises would take place far from Bulgaria's territorial waters. "Although these actions do not constitute a formal violation of international legal norms, they are not considered good practice," the defence ministry said.

A day earlier, NATO said Russia's new Black Sea warning zone, within Bulgaria's exclusive economic zone, had created new risks of miscalculation and escalation, as well as serious impediments to freedom of navigation.