Romania has repeatedly seen violations of its airspace, including drone fragments falling onto its territory, following Russia's invasion of Ukraine in 2022.

In the latest incident, two German fighter jets serving in the NATO country were monitoring the air situation just after midnight at the border with Ukraine, following Russian air strikes, the Romanian defence ministry said in a press release.

A phone alert was sent out to residents of southeastern Tulcea county, and "for a few minutes, the signal of a drone was detected, which had entered approximately eight kilometres (five miles) into national airspace" before it disappeared from the radar.

The drone then "reappeared intermittently on radar for approximately 12 minutes," the ministry said, with an alert also being sent to residents in southeastern Galati county.

"Two more aircraft, F-16s of the Romanian Air Force, were scrambled," the ministry added, saying that "no cases of aircraft impact with the ground have been reported".

"Teams of specialists are ready to begin ground searches," it added.

On Monday, Romania ordered the evacuation of two villages -- the first such evacuation in the country as a result of the war in Ukraine -- after a Russian drone attack across the Danube river in neighbouring Ukraine set fire to a ship carrying liquefied petroleum gas.

The evacuation order was lifted on Tuesday.