Belarus has summoned the Ukrainian ambassador, saying it had shot down a Ukrainian air defence missile in Belarusian territory.
It happened during one of Russia’s heaviest aerial onslaughts against Ukraine so far.
Footage from Belarusian state television appeared to show missile debris in a field.
A Ukrainian military spokesman said the incident was “the result of air defence”.
Belarus – a key ally of Russia – demanded that Kyiv carry out a full investigation, after the S-300 rocket came down near the border.
It is the first time Belarus has shot down a Ukraine missile since the Russian invasion in February.
The Ukrainian ambassador was called in to the foreign ministry in Minsk.
“The Belarusian side views this incident as extremely serious,” spokesperson Anatoly Glaz told the Reuters news agency.
He called on Ukraine to “take comprehensive measures to prevent the recurrence of such incidents in the future”.
And in a press briefing on Friday, the Kremlin said it was greatly concerned by the incident.
Ukraine’s defence ministry said it was ready to invite experts from countries that do not support “the terrorist state of Russia” to help investigate of the incident, adding that it reserved “the unconditional right to the defence and protection of its own sky”.
It said it did not rule out a “deliberate provocation” in which Moscow launched its cruise missiles on a path where they would be intercepted over Belarusian territory.
Belarus said the missile came down near the village of Harbacha in the Brest region, around 15 km (9 miles) from the border with Ukraine.
A witness told Radio Free Europe: “Our windows were shaking and the house was vibrating from what must have been a sound wave.”
The military commissar of the Brest region, Oleg Konovalov, sought to downplay the incident.
“Unfortunately, these things happen,” he said in a video posted to social media.
He compared it to an incident last month when a missile landed in Poland, killing two people. Nato chief Jens Stoltenberg said Kyiv’s air defences were “most likely” to blame, but Ukraine denied this.
Belarus has very close ties with Russia, and allowed Moscow to use its territory in February to start the invasion of Ukraine.
Ukrainian officials recently claimed that Russia could be planning a new offensive from Belarus, but Western governments say they have seen no evidence of that.
Meanwhile, Ukrainian President Volodymyr Zelensky said most regions were without power in freezing conditions following a huge wave of Russian missile attacks.
Moscow fired scores of missiles at cities across Ukraine on Thursday, including in Kyiv, Kharkiv, Lviv and Odesa.
Mr Zelensky warned that Moscow “is only driving itself deeper into a dead end”.
“The status of the largest terrorist in the world will have consequences for Russia and its citizens for a long time. And each missile only confirms that all this must end with a tribunal. This is exactly what will happen,” he said.