Fighters from the Wagner group have arrived in Belarus from Russia, Ukrainian and Polish officials said on Saturday, a day after Minsk said that the mercenaries were training the country's soldiers southeast of the capital.

"Wagner is in Belarus," Andriy Demchenko, a spokesman for the Ukrainian border agency, said in a statement posted on the Telegram messaging app. He added that the movement of "separate groups" from Russia has been observed in Belarus.

Some Wagner fighters have been in Belarus since at least Tuesday, two sources close to the fighters told Reuters.

The Belarusian defence ministry released a video on Friday, showing what it said were Wagner fighters instructing Belarusian soldiers at a military range near the town of Osipovichi.

Wagner's move to Belarus was part of a deal that ended the group's mutiny attempt in June after taking control of a Russian military headquarters, marching on Moscow, and threatening to tip Russia into civil war, President Vladimir Putin said.

Wagner chief Yevgeny Prigozhin has not been seen in public since he left the southern Russian city of Rostov-on-Don late on June 24.

Poland's Deputy Minister Coordinator of Special Services Stanislaw Zaryn said that Warsaw also has confirmation of Wagner fighters' presence in Belarus.

"There may be several hundred of them at the moment," Zaryn said on Twitter.

Poland said earlier in July that it was bolstering its border with Belarus to address any potential threats.

The Belarusian Hajun project monitors military activity in the country and is viewed as an extremist organization by Belarusian authorities. It said a large caravan of at least 60 vehicles - including pickups, vans and buses- entered Belarus overnight Friday from Russia.

Hajun said it appeared to be a Wagner group headed to Tsel in central Belarus.

Reuters could not independently verify the Belarusian Hajun report. There was no immediate comment from Russia or Belarus on the reports.

(Writing by Lidia Kelly in Melbourne and Mark Trevelyan in London)