The attack rattled windows in the Romanian border village of Plauru across the river and Popescu's trailer shook.

"I counted five explosions, three very powerful ones," said Popescu, who has been bringing his beehives to Plauru from central Romania for six years.

Ukraine said Russian drones had detonated on the territory of NATO member Romania, a rare report of stray weapons from the war hitting a neighbouring member of the Western military alliance.

Romania strongly denied it had been hit, but the attacks on Ukraine's river ports, just hundreds of metres from the Romanian border, have increased security risks for NATO which has a collective defence commitment.

"I can tell you no piece, no drone and no part of a device landed in Romania," President Klaus Iohannis told reporters on Tuesday. "We have total control over our national space...

"But, yes, we are concerned, because these attacks are taking place very close to the Romanian border. I was ... told today there were verified attacks 800 metres from our border, so very, very close."

Ukraine has reported suspected Russian weapons flying over or crashing into neighbours, including NATO members, several times during the war.

In the most dramatic incident, two people were killed in Poland by a missile that fell near the border last November. Poland and NATO allies later said it was a misfired Ukrainian air defence missile.

The Danube river is now Ukraine's main route out for grain after access to its own Black Sea ports was cut off after Russia in mid-July left a safe passage grain corridor agreement brokered by the United Nations and Turkey. Since then, Russia has attacked Ukraine's river ports Izmail and Reni repeatedly.

In Plauru, the few remaining residents can hear the bomb sirens which ring out in Izmail several times a week.

"We ignore them during the day, but our souls tense up when they ring at night," said resident Daniela Tanase.

Despite government assurances that Romania is protected by its NATO membership, Popescu cannot take heart.

"I mean, I don't feel safe here. I am on the Romanian shore and I do not feel safe."

(Reporting by Andreea Campeanu; Additional reporting by Luiza Ilie; Writing by Luiza Ilie; Editing by Nick Macfie)

By Andreea Campeanu and Luiza Ilie