NATO condemned Moscow's "recklessness" on May 29 as it confirmed an apartment building in Romania was struck by a drone as Russia attacked Ukrainian infrastructure near the border.
"NATO will continue to strengthen our defences against all threats, including drones," said NATO spokesperson Allison Hart on social media, adding that alliance chief Mark Rutte was in touch with Romanian authorities.
NATO-member nations that border the warring sides are facing increasing risks as overnight air barrages intensify, with Latvia's government collapsing in recent weeks due to scrutiny of its defense capacities following a stray Ukrainian drone incursion.
"During the night of May 28-29, the Russian Federation resumed drone attacks on civilian and infrastructure targets in Ukraine, near the river border with Romania," the Romanian defense ministry said.
"One of these drones entered Romanian airspace, was tracked by radar as far as the southern part of the city of Galati, and crashed onto the roof of an apartment building, with the impact triggering a fire," it said.
The emergency services said two people with abrasions required medical treatment and that the fire had been extinguished.
Romania's foreign ministry accused Russia on May 29 of a "serious and irresponsible escalation."
"This incident represents a serious and irresponsible escalation on the part of the Russian Federation," the ministry statement said, adding Bucharest had informed the NATO secretary general and "requested measures to accelerate the transfer of anti-drone capabilities to Romania".
Drone incursions in Romania have been detected dozens of times since the start of the Russian offensive against Ukraine in 2022, but the latest incident was the first time one had hit a residential building.
Two F-16 fighter jets were scrambled after the drones were detected in Romanian airspace, the defense ministry said.
A nationwide air raid alert had been issued in neighbouring Ukraine overnight in anticipation of Russian strikes, with at least two people wounded in the attack in southern Ukraine's Zaporizhzhia, according to local authorities.
EU chief Ursula von der Leyen said that Russia's "war of aggression has crossed yet another line."
"We stand in full solidarity with Romania and its people," she wrote on social media. "As we continue strengthening our security and deterrence, especially on our Eastern border, we will keep increasing the pressure on Russia."
The overnight incursion followed a massive series of Russian strikes on Ukraine on Saturday in one of the most severe attacks since the start of the war.
Russia has been threatening to escalate bombardments in retaliation for a Ukrainian strike that, according to Moscow, killed 21 people at a school in occupied Ukrainian territory.
Moscow announced Monday it had started a campaign of "systematic" strikes on Kiev, after battering Ukraine with hundreds of drones and a hypersonic missile over the weekend, and called for diplomats and foreigners to leave the Ukrainian capital.
Ukrainian President Volodymyr Zelensky has urged the United States to provide more ammunition for its Patriot air defense systems to counter Russian ballistic missiles, according to a document reviewed by AFP this week.
NATO-member states bordering Ukraine or Russia, including Romania, Latvia, Estonia and Poland are increasingly exposed to incursions into their territory by drones from both warring sides.
Latvia, which borders Russia, appointed a new government on Thursday, two weeks after the collapse of the previous administration due to a row over stray Ukrainian drone incursions, which exposed the weaknesses of the country's air defenses.
The former Latvian prime minister had accused her defense minister of not deploying anti-drone defenses fast enough to parry two wayward Ukraine attack drones, which are thought to have been knocked off course by Russian jamming.
The drones caused minimal damage but sparked widespread concern in the former Soviet republic, which is now a member of NATO and the European Union.
Zelensky has offered to send experts to Latvia to help it boost its air defenses.
Diplomatic efforts to end the four-year war have stalled since Washington's attention was diverted in February to its conflict with Iran.
The EU's top diplomat on Thursday ruled out Europe acting as a "neutral mediator" between Ukraine and Russia, after foreign ministers from the bloc's 27 countries debated their terms for possible talks with Moscow.
Ukraine has pushed for Europe -- sidelined until now by Washington -- to play a bigger role and suggested nominating a representative for talks but the EU's foreign policy chief said it could not be a "neutral mediator" given its support for Ukraine.
