Ukraine has denied targeting Bulgaria after a drone entered the country’s airspace on Saturday and exploded close to an international gas pipeline shortly after crossing the border from Romania. There were no casualties or damage.

The defense ministry in Sofia, Bulgaria’s capital, said there were indications the drone was the kind used by Ukrainian armed forces and that the incident was likely accidental. Bulgaria and Romania are both members of NATO and the European Union.

A Ukrainian foreign ministry spokesperson said the Ukrainian military was looking into the incident but that it did not deliberately target Bulgaria. Heorhii Tykhyi said on Saturday that Ukraine and Bulgaria were in “close contact” to establish the circumstances of the incident.

Bulgaria’s Prime Minister Rumen Radev said the drone exploded just after 5 a.m. about 109 yards inside Bulgarian territory. It was not detected or identified while flying through either Romanian or Bulgarian airspace.

Radev said the unmanned aerial vehicle exploded around 200 meters (218 yards) from a Romanian compressor station, and about 0.6 miles from Bulgaria’s compressor station on the Trans-Balkan gas pipeline.

“We will continue to monitor the situation and strengthen surveillance. Following this incident, we will also redeploy border police anti-drone detection and countermeasure capabilities from the border with Turkey to the border with Romania,” Radev said.

The drone appeared to have carried a significant amount of explosives, given the strength of the blast and the column of black smoke, Stoyanov said, adding that such drones are difficult to detect because of their small size and low flight altitude.

Drones straying into NATO airspace have not been uncommon since Russia’s full-scale invasion of Ukraine. In June, two NATO fighter jets shot down a drone that entered the Latvia’s airspace, while the two other Baltic states, Estonia and Lithuania, which all share a border with Russia, reported drones crashing in their territory. 

Moldova and Romania, two countries which neighbour Ukraine, have also reported separate incidents of drones crashing in their borders. Officials blamed the inadvertent incursion on Russian electronic warfare.