Romania reacted with fury on Wednesday after debris from a Russian killer drone was found on its territory near the Danube river, following Moscow’s assault on a Ukrainian port.

Russian drone debris landing in Romania would be “a serious violation of the sovereignty and territorial integrity of Romania,” the country’s President Klaus Iohannis said Wednesday.

Romanian Defense Minister Angel Tîlvăr confirmed that pieces of the attack drone were discovered in the country, which is a member of the NATO military alliance.

“We covered a very large area, including the area about which there were discussions in the public space and I confirm that pieces were found that could be a drone,” Tîlvăr said Wednesday, according to local media outlet Antena 3 CNN.

Russia has been bombarding Ukrainian ports on the banks of the Danube since President Vladimir Putin pulled out of the Black Sea grain deal, with missiles and drones frequently landing near Romania.

Ukraine on Monday said that Russian debris fell on Romanian territory after an attack, which was at first vigorously denied by Iohannis.

After Tîlvăr’s confirmed that drone wreckage had been discovered, Iohannis said he was informed “in real time” and that he requested an “urgent and professional investigation” into the origin of the debris and how it landed on Romanian territory. 

If the debris is confirmed to be Russian, it would be “completely unacceptable,” Iohannis added.

Last November, NATO held crisis talks after a missile landed in Poland, though U.S. President Joe Biden later said it was unlikely to have been fired from Russia.

As NATO members, Romania and Poland are protected under article 5 of the military alliance’s treaties, which says that an attack on one member will be considered an attack on all members.

Iohannis said the country is “on alert” and in contact with NATO allies.

“Within NATO, we are very well defended,” he said. “And Romania benefits from extremely strong security guarantees, the strongest in our entire history.”