Two commercial ships were struck by drones off the coast of Bulgaria on Tuesday, one of which sank, presenting a “challenge” to NATO, The Wall Street Journal reported. The “challenge” for the alliance is to respond to these and other such incidents without activating Article 5 of the NATO treaty. “The alliance is pressing its members to call out Russia for such attacks when there is enough evidence to point to the Kremlin’s involvement, while seeking to avoid a direct confrontation between the military alliance and Moscow,” the Journal said.

NATO Secretary General Mark Rutte said that he had spoken with Bulgarian Prime Minister Rumen Radev but didn’t apportion blame for the strikes. He reportedly called on the both EU and NATO to join forces to stop attacks on merchant ships in the Black Sea after also speaking with Ursula von der Leyen, the government press office said.