(Reuters) -Belarus protested to Ukraine's ambassador on Thursday after saying it had downed a Ukrainian S-300 air defence missile in a field, during one of Russia's heaviest aerial onslaughts against Ukraine since the start of the war in February.
The military commissar of the Brest region, Oleg Konovalov, played the incident down in a video posted on social media by the state-run BelTA news agency, saying locals had "absolutely nothing to worry about. Unfortunately, these things happen".
He compared the incident to one in November, when an S-300 believed to have strayed after being fired by Ukrainian air defences landed on the territory of NATO-member Poland, triggering fears of an escalation that were rapidly defused.
Nevertheless, the Ukrainian ambassador was summoned to the foreign ministry in Minsk to receive a formal protest.
"The Belarussian side views this incident as extremely serious," spokesperson Anatoly Glaz said.
"We demanded that the Ukrainian side conduct a thorough investigation, ... hold those responsible to account and take comprehensive measures to prevent the recurrence of such incidents in the future."
Ukraine's defence ministry said it was ready to invite authoritative experts from countries that do not support "the terrorist state of Russia" to participate in an investigation of the incident, which it said resulted from the repulsion of a "barbaric" Russian missile attack that hit civilian targets.
"The Ukrainian side, reserving the unconditional right to the defence and protection of its own sky, at the same time is ready to conduct an objective investigation in Ukraine," the ministry said in a statement.
It said it did not rule out a "deliberate provocation" in which Russia launched its cruise missiles on a path where they would be intercepted over Belarusian territory.
A Ukrainian military spokesman also in effect acknowledged that the missile was a Ukrainian stray, saying the incident was "nothing strange, a result of air defence" and something that "has happened more than once".
The S-300 is a Soviet-era air defence system used by both Russia and Ukraine.
Belarus said the missile had come down near the village of Harbacha in the Brest region, some 15 kms (9 miles) from the border with Ukraine, at around 10 a.m. (0700 GMT).
"Fragments were found in an agricultural field ... the wreckage belongs to an S-300 anti-aircraft guided missile fired from the territory of Ukraine," the Belarus defence ministry said.
The missile strayed as Russia was launching its latest wave of missile attacks on cities across Ukraine, in which the defence ministry said more than 18 residential buildings and 10 critical infrastructure targets in 10 regions were destroyed.
BelTA published photos and video of what it said were parts of an S-300 missile lying in an empty field.
Belarus allowed Moscow to use its territory in February to start the invasion of Ukraine, and there has been a growing flurry of Russian and Belarusian military activity in Belarus in recent months.
Minsk has, however, insisted that it is not participating in the war, and will not participate unless its own security is threatened by Ukraine or Ukraine's Western allies.