Lithuania's Foreign Ministry accused Belarus on Friday of an attempt to interference with diplomatic mail and summoned Minsk's ambassador to lodge a protest over the matter.
A Lithuanian courier who set out by car from Vilnius on Friday carrying documents intended for the Lithuanian Embassy in Belarus was asked by Belarusian border guards to open the sealed container, the ministry said.
It said the courier declined to open the container and was then turned back from the border, and that the incident violated rules governing international diplomatic activity.
The Belarusian Embassy in Vilnius did not immediately respond to requests for comment.
Britain, the United States and the European Union all imposed bans and asset freezes on some Belarus officials after an election last year that the opposition says was rigged.
Relations deteriorated further last month after a Ryanair plane overflying Belarus on an Athens-to-Vilnius flight was forced to land in Minsk, where a dissident Belarusian journalist and his girlfriend aboard were arrested.
Belarusian opposition leader Sviatlana Tsikhanouskaya, who lives in exile in neighbouring Lithuania, has repeatedly called for more sanctions on the government of Belarus President Alexander Lukashenko.
(Reporting by Andrius Sytas, editing by Terje Solsvik and Mark Heinrich)
