Polish and allied aircraft were also deployed in a preventive operation in Poland's airspace because of a threat of drone strikes in neighbouring Ukraine, and the airport in the eastern Polish city of Lublin was temporarily closed.
Ukrainian President Volodymyr Zelenskiy said on social media platform X that data showed the drone breached about 10km into Romanian territory and operated in NATO airspace for around 50 minutes.
"It is an obvious expansion of the war by Russia – and this is exactly how they act," he said.
Romania, a European Union and NATO state which shares a 650-km (400-mile) border with Ukraine, has had Russian drone fragments fall onto its territory repeatedly since Russia began waging war on its neighbour.
On Saturday, Romania scrambled two F-16 fighter jets and warned citizens in the southeastern county of Tulcea near the Danube and its Ukrainian border to take cover, the defence ministry said in a statement.
The incidents came after multiple Russian drones crossed into Poland on Wednesday, prompting NATO to send fighter jets to shoot them down.
The Polish military's operational command posted on X on Saturday afternoon that ground-based air defence and reconnaissance systems were on high alert.
The alert lasted around two hours.
It stressed that "these actions are preventive in nature," and were aimed at securing Poland's airspace and protecting the country's citizens.
It cited a threat of drone strikes in regions of Ukraine bordering Poland, but didn't immediately give further details.
Prime Minister Donald Tusk also posted that "preventive air operations" had begun in Polish airspace because of the threat posed by Russian drones operating over nearby areas of Ukraine.
Russia has said it didn't target Poland on Wednesday, and Moscow's ally, Belarus, said that the drones went astray because they were jammed.
But European leaders have expressed certainty that the incursions were a deliberate provocation by Russia.
US Secretary of State Marco Rubio on Saturday said the incursion of Russian drones into Polish airspace this week was unacceptable, but that it remained unclear if Russia had deliberately sent the drones intoPolish territory.
Meanwhile, Ukraine will need at least $US120 billion ($A182 billion) for its defence in 2026 as the war with Russia drags on, Defence Minister Denys Shmyhal said on Saturday.
Shmyhal said the funding was essential to maintain defence lines, produce more drones and other weapons, protect skies, and deter any further Russian aggression.
"The economy of the war demonstrates that if we spend less money than Russia, then we begin paying with our territories and, most importantly, with our lives," Shmyhal told an annual conference in Kyiv.
with Reuters
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