NATO member Romania scrambled fighter jets overnight after Russia attacked Ukrainian infrastructure close to the alliance’s border, according to Bucharest.

On Sunday, Romania detected “groups of drones” heading across the Black Sea towards Ukraine, close to Romanian airspace, after 23:30 local time, the country’s Ministry of Defense announced. Authorities launched an air alert in Romania’s Tulcea county, the ministry said, as Moscow “carried out a new series of drone strikes on civilian and infrastructure targets in Ukraine.”

The city and wider region of Tulcea is just across the border from the Ukrainian port of Izmayil, which Russia has repeatedly attacked. The Danube River marks the border between the two countries, just north of Tulcea.

Two F-16 fighter jets of the Romanian Air Force took off from the Borcea Air Base, in the southwest of Tulcea, “to monitor the situation”, the ministry said. The airbase is east of the capital.

The air alert was ended at 12:45 p.m. and the planes returned to their base, the government added.

Newsweek contacted the Russian Ministry of Defense for comment via email.

Russian the drones crossed NATO territory several times since the beginning of the full-scale war in Ukraine in February 2022, with fragments found in Romania several times.

Drones wandering in nato territory were not considered intentional attacks, but the incidents have fueled tensions between the alliance and Moscow at their worst in decades.

This type of incursion was “unthinkable” three years ago, but it is “now treated as routine,” Lithuanian Foreign Minister Gabrielius Landsbergis said in September.

Oleh Kiper, head of Ukraine’s Odesa region, which includes Izmayil, warned residents of the port city to shelter in place overnight. The Ukrainian Air Force said it detected Russian attack drones over Odesa, heading west from the area north of Izmayil.

Kiper said in late September that a “large-scale” Russian drone attack on Izmayil had killed three people.

Romania will investigate Monday morning to see if any of the drones landed on NATO territory, the government said.

On October 23, Romania said it did detected at least one unidentified “air target”. along the border between Romania and Ukraine. Just days earlier, radars in Bucharest had detected a “small aerial target” over the Black Sea, crossing Romania’s airspace about an hour and a half after it was first detected. Another object had violated the Romanian airspace two days ago.

The Ministry of Defense in Bucharest published a draft law at the end of October, which means it could intercept or shoot down drones that illegally enter its airspace.