Ukraine said the attack early on Wednesday had struck the ports of Reni and Izmail, which lie across the Danube from Romania, damaging warehouses used for grain cargoes, oil storage tanks and administrative buildings.
The attacks, which have intensified since mid-July when Moscow abandoned a deal that lifted a de facto Russian blockade of Ukraine's Black Sea ports, have increased security risks for NATO whose members have a mutual defence commitment.
"The crew of an IAR 330 Puma helicopter of the Romanian Air Force ... (identified) fragments that could have come from a drone, dispersed over an area of several dozen metres," the defence ministry said in a statement.
It said that the fragments were found near the villages of Nufarul and Victoria in Tulcea county.
If confirmed, it would be the third time that such fragments have been found on Romanian territory in recent days.
Preliminary analyses of the first two drone fragments have shown they did not explode in Romania and were not carrying explosives.
"No one has attacked us," Prime Minister Marcel Ciolacu told reporters. "Some pieces of a drone that was hit by the Ukrainian army fell. It did not have explosives, or anything damaging."
The defence ministry has started building two bomb shelters in the small border village of Plauru, where the first two fragments were found.
It said it would deploy additional troops in the area, and increase patrols and observation points, while local residents will receive notification alerts to take shelter.
(Reporting by Alan Charlish in Warsaw and Luiza Ilie in Bucharest; Editing by Kevin Liffey and Christina Fincher)
