A Ukrainian S-300 missile fell onto the territory of Belarus on Thursday, Belarus' state-run BelTA news agency reported.
The Minsk defence ministry was investigating whether Belarus' air defence systems had shot down the rocket or it was a misfire.
The incident occurred between 10 and 11 a.m. (0700-0800 GMT) - around the time Russia was firing scores of missiles towards Ukraine.
BelTA said the missile that landed in Belarus had come from Ukrainian territory. It published a photo of what it said were parts of a S-300 missile lying in an empty agricultural field.
BelTA said there was no information about casualties.
The S-300 is a Soviet-era air defence missile that is used by both Russia and Ukraine. In November, an S-300 believed to have been fired by Ukrainian air defences landed in Poland.
Kyiv has used the system to intercept incoming Russian missiles, while Russia has appeared to use repurposed S-300 missiles to attack ground targets - something that military analysts say could be a sign that its missile stocks are dwindling.
Belarus allowed Moscow to use its territory as a launchpad for the invasion of Ukraine in February, but has insisted that it is not participating in the war, and will not participate.
