The Russian embassy in Warsaw said it had not been informed of the matter and questioned the truth of the announcement.

"We have detained a Russian deserter. He had his military papers on him," border guard spokeswoman Katarzyna Zdanowicz told AFP.

"The individual is a 41-year-old man who illegally crossed the Belarusian border into Poland, in the northeast," she said, without providing further details.

The soldier "had fought in Ukraine", Zdanowicz told TVN24 television.

She was unable to say whether the desertion into Poland was the first case of the sort since Russia's invasion of Ukraine began in 2022.

Polish media reported that the man wore civilian clothes and was unarmed.

The Russian embassy said it "has not received any information from Polish officials on the detention".

"The Polish side is required to inform the embassy" of such things under the bilateral consular convention, it added, quoted by Russia's TASS state news agency.

"In view of this, we intend to ask the Polish foreign ministry whether a Russian citizen was really detained."

Belarus and Russia are members of the Collective Security Treaty Organisation alliance that is made up of several ex-Soviet countries.

Moscow and Minsk are close defence partners, and Belarusian President Alexander Lukashenko allowed Russian forces to attack Ukraine from his territory in February 2022, when tens of thousands of Russian troops were stationed in his country.

Relations between Warsaw and Minsk have been strained for years, due to a political crackdown in Belarus and a tussle over migrants.

They have sunk to new lows since Lukashenko backed Russia's invasion of Ukraine.

The border guard announcement comes as Poland reels from the revelation this week that a Polish judge had fled to Belarus and reportedly asked for political asylum over spying allegations he denies.

Warsaw has since opened an espionage probe as the judge, Tomasz Szmydt, had access to classified information in Poland.

burs-amj/tw