Stoltenberg said Romania on Wednesday informed its NATO allies of the discovery and that it "demonstrates the risk of incidents and accidents".

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

"Regardless of that outcome, what we have seen of course is a lot of fighting and also air attacks close to NATO borders."

Romanian President Klaus Iohannis on Wednesday called for an "urgent investigation" into debris, believed to be the remains of a drone, which was discovered on its soil following attacks in neighbouring Ukraine.

The revelation came after NATO member Romania repeatedly rejected claims by Kyiv that Iranian-made Russian drones fell and detonated on Romanian territory during a strike on the Ukrainian port of Izmail on Sunday night.

US Secretary of State Antony Blinken, who was visiting Ukraine, discussed the ongoing investigation in a telephone call with Romanian Foreign Minister Luminita Odobescu, the State Department said.

The two also discussed "additional cooperation to preserve airspace security," State Department spokesman Matthew Miller said.

Since Russia's all-out invasion last February, NATO has been focused on preventing the war in Ukraine from spilling over onto its territory.

Any attack on one of its members could drag NATO into a potentially nuclear conflict with Moscow owing to the US-led alliance's joint defence pact.

Fears soared of a potential flare-up in November last year when a missile from across the border killed two people in Poland.

NATO said the projectile was a Ukrainian air defence missile and not fired by Russia.