"While carrying out missions in the Black Sea operational zone, one of the Ukrainian Navy's uncrewed surface vessels lost control due to enemy electronic warfare measures and drifted toward the Romanian coast," the Ukrainian Navy said in a statement.
The Navy added that Ukraine had provided Romanian naval forces with the necessary information to help prevent civilian casualties.
Earlier on June 5, a maritime drone exploded in Romania's port of Constanța. Romania's Ministry of National Defence said the drone may have been Ukrainian.
The ministry said the drone self-destructed.
Commenting on the explosion of the uncrewed vessel in the port and the recent crash of a Russian drone into a residential building in Galați, Romanian President Nicușor Dan said such incidents were direct consequences of Russia's war against Ukraine.
"Given the military conflict on our border, it is obvious that the security environment we are living in is sensitive, and we will maintain a high level of vigilance. Such particularly serious situations are direct consequences of the aggressive war launched by Russia against Ukraine," he wrote on X.
European Commission President Ursula von der Leyen also said the incidents were consequences of Russia's war.
"One week after a drone struck an apartment building in Galați, a maritime drone reached the port of Constanța today. This is a direct consequence of Russia's war against Ukraine. It is increasingly becoming a direct threat to countries on our eastern border," she wrote on X.
Von der Leyen said the European Union stands in full solidarity with any member state facing such threats and stressed that the response must match the urgency of the situation.
Russian drone strike on residential building in Romania on May 29
Overnight into May 29, a drone carrying at least 30 kilograms (66 pounds) of explosives crashed onto the roof of an apartment building in the Romanian city of Galați, injuring two people.
Romania's Defense Ministry later confirmed the drone was a Russian Geran-2. Romania's Foreign Ministry described the incident as a "serious escalation" by Russia and announced plans to invoke Article 4 of the NATO treaty for consultations among allies. NATO, in turn, condemned Russia's "irresponsible actions."
Romanian President Nicușor Dan later announced that Russia's consulate in Constanța would be closed and the consul declared persona non grata.
Ukrainian President Volodymyr Zelenskyy subsequently spoke with Dan and said Ukraine would send specialists to Romania to help strengthen air defense capabilities.
Dan later said the Shahed drone that struck Galați had likely been hit by Ukrainian air defenses and changed course.