Ukraine's foreign ministry spokesman said Russian drones fell and detonated on Romanian territory on Monday during an overnight attack on Ukrainian port infrastructure on the Danube river.
Romania is a member of the NATO military alliance.
"According to Ukraine's state border guard service, last night, during a massive Russian attack near the port of Izmail, Russian 'Shakheds' fell and detonated on the territory of Romania," Ukrainian official, Oleg Nikolenko, wrote on Facebook.
"This is yet another confirmation that Russia's missile terror poses a huge threat not only to Ukraine's security, but also to the security of neighboring countries, including NATO member states," he said.
Nikolenko published a photo showing the flames of an explosion on the opposite bank of the Danube river.
An Ukrainian industry source told Reuters that two Russian drones had fallen on the Romanian side of the Danube.
Ukraine's Danube ports, Reni and Izmail, accounted for around a quarter of grain exports before Russia pulled out of a UN-backed deal that provided safe passage for the export of Ukrainian grain via the Black Sea.
The Danube ports have since become the main route out of Ukraine, with grain also sent on barges to Romania's Black Sea port of Constanta for shipment onwards.
Romania denies detonation
Russia's overnight attack on Ukrainian port infrastructure on the Danube river on Monday did not generate direct military threats to neighboring NATO state Romania's territory, Romania's defense ministry said in a statement.
Earlier, Ukraine's foreign ministry said drones had detonated on Romanian territory. The Romanian defense ministry's statement "categorically denied" the reports while condemning the Russian attack.