A commercial ship sank and its crew vanished Tuesday after an apparent coordinated air-and-sea drone attack off NATO member Bulgaria, as Ukrainian President Volodymyr Zelenskyy blamed Russia before warning Moscow was preparing a broader "massive strike."

A second vessel was also heavily damaged and set ablaze inside Bulgaria’s exclusive economic zone, with a drone expert warning the dual precision strikes signaled a dangerous escalation and possible "larger invasion posture" by Russia.

"They were likely using the Geran-4 aerial drone to take the vessel out or the Maran surface vehicle to take it out," Cameron Chell, founder of drone manufacturer Bulgaria’s EEZ, calling the attack a "gross, flagrant violation of international law," according to Reuters.

The sunken vessel, identified by British maritime risk group Vanguard as the cargo ship Alfa Watan, suffered catastrophic structural damage before submerging with an undisclosed cargo and an unknown number of crew members aboard.

A nearby Bulgarian ferry, the Dioscuria, diverted to the scene after witnessing an explosion and receiving distress signals, Reuters reported.

Rescuers found an overturned lifeboat, life rafts and life jackets floating in the debris field.

Search-and-rescue operations involving the Bulgarian Navy, Air Force and Border Police remained underway to locate the missing crew and monitor for environmental threats, the outlet said.

Chell suggested the apparent coordination between the attacks was particularly significant.

"Not only are you using pretty sophisticated equipment, but you're also doing it in a coordinated manner," Chell said.

"These two ships were going opposite directions. One was actually just coming, and had left Ukraine the day before," he said.

"These things coordinated and were struck relatively at the same time. So another level of coordination has unfolded here," Chell added.

The second ship, identified by Vanguard as the general cargo vessel ABLE, caught fire after being struck while carrying grain.

A Bulgarian ferry crew evacuated all 18 seafarers aboard — 11 Turkish nationals and seven Indian nationals.

Two crew members suffered severe injuries and were airlifted to a medical facility in Varna, while the remaining 16 were transported safely to shore.

Radev convened an emergency government session following the strikes. While acknowledging it remained unclear whether the drones originated from Russia or Ukraine, he warned of the widening scope of the war.

"Such an attack within the Bulgarian exclusive economic zone is absolutely unacceptable," Radev told reporters.

"If this war is not ended in time, it will continue to expand its spatial scope and increase the risk of even a global conflict."

Chell said striking the two ships inside Bulgaria’s exclusive economic zone could also have the effect of disrupting a crucial shipping corridor and creating fear among commercial operators.

"It all adds up to a perfect little scenario for the Russians to create chaos there, not just to disrupt the lane of economics for the region and in particular for Ukraine — a lifeline," Chell said.

"The Russians are really taking a playbook out of the Iranians who have basically created enough chaos in the Strait of Hormuz where the operators can't take the chance to put their ships through there," he added.

The strikes came amid a surge in attacks on commercial shipping in the Black Sea, including an incident off Romania’s coast Monday in which two people died in a ship fire.

Zelenskyy later warned that Ukrainian intelligence had obtained information indicating Russia was preparing a major new assault.

"We’ve got updated intel on Russia’s preparations for a massive strike," Zelenskyy wrote on X. "I ask you, please, to take care of yourselves and respond to missile threat alerts."

"Ukrainian services and our Air Force are ready to respond — as much as our defenses allow," Zelenskyy continued. "And a lot depends on our partners — every country in the world that has the defensive capabilities we need and can provide them to us."

"It’s very important that there be the political will to do this — particularly in the United States and in Europe," he added.