MINSK, Dec 29 (AFP) - Belarus has protested to Ukraine’s ambassador after saying it had downed a Ukrainian S-300 air defence missile in a field, during one of Russia’s heaviest aerial onslaughts since the start of the war.

The military commissar of Belarus’s southwestern region of Brest, Oleg Konovalov, played Thursday’s 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 said.

Konovalov 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 Belarusian side views this incident as extremely serious,” Anatoly Glaz, a spokesman for the ministry, said.

“We demanded that the Ukrainian side conduct a thorough investigation … [and] hold those responsible to account and take comprehensive measures to prevent the recurrence of such incidents in the future.”

A Ukrainian military spokesman in effect acknowledged 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”.