NATO member Romania said on Sunday it had summoned the Russian embassy's charge d'affaires following the discovery of fragments of a drone "similar to those used by the Russian army".
Romanian soldiers found the fragments in the area of Plauru in Tulcea county across the border from Ukraine on Saturday.
State secretary Iulian Fota met the Russian diplomat the same day to transmit "Romania's protest against the violation of the Romanian airspace," the foreign ministry said.
Fota "firmly asked the Russian side to stop the actions against the Ukrainian population and infrastructure, including those that would threaten in any way the safety and security of the Romanian citizens in the region."
Romanian President Klaus Iohannis and NATO chief Jens Stoltenberg said on Saturday that they discussed the discovery during a phone call.
Stoltenberg said on X, formerly Twitter, there was "no indication of intent to hit NATO, but the strikes are destabilising."
He also welcomed the US decision to deploy more F-16 fighter jets for NATO air policing.
Iohannis said the discovery indicated "there took place an absolutely unacceptable breach of the Romanian sovereign airspace".
Romania has beefed up measures to strengthen monitoring and airspace security following repeated Russian attacks across the border on Danube ports and infrastructure.
Drone debris had already been found in the Plauru area earlier this week, some kilometres away from Saturday's find, which is being analysed.
Wednesday's revelation of the first discovery of possible drone debris came after Romania repeatedly rejected claims by Kyiv that Iranian-made Russian drones fell and detonated on Romanian territory during a strike on the Ukrainian port of Izmail on Sunday night.
Since Russia's all-out invasion of Ukraine last February, NATO has been focused on preventing the war from spilling over onto its territory.
Russia has ramped up attacks on Ukraine's southern Odesa and Mykolaiv regions, home to ports and infrastructure vital for agriculture exports following the collapse of a deal allowing grain shipments from Black Sea ports.
