Romania's defence ministry said a woman and a child suffered minor injuries in Galati near the border with Ukraine overnight after radar tracked a Russian drone entering Romanian airspace and reaching the southeastern city.
Photos from the scene showed charred and damaged brickwork on the roof of the 10-storey apartment block which Romanian authorities said was hit by a drone that exploded on impact, causing a fire in a top-floor flat.
Russia did not immediately comment.
It was the first time a densely populated area in a NATO country had been hit causing injuries during Russia's war in Ukraine, and the incident is likely to increase tensions on the alliance's eastern flank as member states worry about the war spilling over their borders.
"Russia's reckless behaviour is a danger to us all," NATO Secretary General Mark Rutte said on X after speaking by phone to Romanian President Nicusor Dan, whose country is in both the North Atlantic Treaty Organisation and the European Union.
"I affirmed that NATO stands ready to defend every inch of Allied territory," he said, without making any mention of triggering NATO's mutual defence clause.
"We will continue to enhance our readiness to deter and defend against any threat, including from drones."
Dan said Russia's behaviour "demonstrates total disregard for international law and for the safety of the citizens of a NATO member state."
Romania, which shares a 650km land border with Ukraine, has experienced Russian drones breaching its airspace 28 times since Moscow began attacking Kyiv's ports across the Danube river, Romania's defence ministry said.
"Romania will take the necessary diplomatic measures in response to this serious violation of international law and its airspace," Romanian Foreign Minister Oana Toiu said on X.
"Romania has informed the Allies and the NATO Secretary General of the circumstances and has requested measures to accelerate the transfer of anti-drone capabilities to Romania."
European Commission President Ursula von der Leyen said Russia had "crossed yet another line".
"We stand in full solidarity with Romania and its people," von der Leyen said in a post on X.
"As we continue strengthening our security and deterrence, especially on our eastern border, we will keep increasing the pressure on Russia."
Romania's defence ministry said it scrambled two F-16 fighter jets and a military helicopter to monitor the attack, adding the pilots were authorised to shoot down any drones.
The residents of border counties Braila, Galati and Tulcea were warned to take cover.
Romanian law allows it to shoot down drones during peacetime if lives or property are at risk, but it has not yet done so.
Ukrainian drones have strayed into Baltic countries' airspace in recent weeks, sowing confusion and raising tensions with Russia.
In Friday's incident, a fire broke out in a 10th-floor apartment after the drone struck the building's roof and exploded, Romania's emergency response agency said. Two people were receiving medical treatment on site, it said, adding 70 people had evacuated.
The drone's entire explosive payload detonated, and the fire was extinguished, the emergency agency said.
The defence ministry said radar lost contact with the drone as it entered Galati as it was flying very low to the ground.
