NATO member Romania on Saturday reported a possible violation of its airspace during overnight drone attacks by Russia on infrastructure in neighbouring Ukraine.

Since exiting a deal allowing safe grain shipments via the Black Sea, Moscow has ramped up strikes on Ukraine's southern Odesa and Mykolaiv regions, home to vital grain-exporting infrastructure.

"Following the detection of groups of drones heading towards Ukrainian territory near the Romanian border" residents in the Tulcea and Galati municipalities were alerted, the defence ministry said in a statement.

"The radar surveillance system ... indicated possible unauthorised entry into national airspace, with a signal detected on a route towards the municipality of Galati," it added.

The defence ministry said that no objects so far appear to have fallen on Romanian territory but the search will continue on Saturday.

Around midnight, residents of Galati and Tulcea, which face the port of Reni in southern Ukraine across the Danube, received alerts warning them to take shelter.

The alert measures were lifted around two hours later.

Earlier this month, Romanian soldiers built air-raid shelters to protect the residents in the eastern Romanian village of Plauru, after drone fragments were found in the area.

Since Russia's invasion of Ukraine in February last year, NATO has been concerned to prevent the war from spilling over onto its territory.