NATO does not have any indication that drone debris found on Romanian territory was caused by a deliberate Russian attack on Romania, its chief said.

Ukraine had said on Monday that drones detonated in Romania during an overnight Russian air strike on a Ukrainian port across the Danube River. Attacks have increased on the site since July when Moscow abandoned a deal that lifted a de facto Russian blockade of Ukraine's Black Sea ports.

Romanian officials initially denied the Ukrainian allegations but changed course on Wednesday, with Defence Minister Angel Tilvar saying that parts of what could be a Russian drone fell on Romanian soil.

"We don't have any information indicating an intentional attack by Russia, and we are awaiting the outcome of the ongoing investigation," NATO Secretary-General Jens Stoltenberg told EU lawmakers.

Stoltenberg stressed the risks Russian air strikes in such close vicinity to the alliance's territory posed.

"It demonstrates the risks of incidents and accidents", he said. "Regardless of that outcome (of the investigation), what we have seen of course is a lot of fighting and also air attacks close to NATO borders."

Russia has carried out long-range air strikes on targets in Ukraine since it invaded its neighbour last year, and Kyiv has reported suspected Russian weapons flying over or crashing into neighbouring countries several times.

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