KYIV, July 16 (Reuters) - Fighters from the Wagner group have arrived in Belarus from Russia, Ukrainian and Polish officials said on Saturday, a day after Minsk said 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 on the Telegram messaging app. He said the movement of "separate groups" from Russia had 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 - when they took control of a Russian military headquarters, marched on Moscow and threatened 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 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 this month it was bolstering its border with Belarus to address any potential threats.

While not sending his own troops to Ukraine, Belarusian President Alexander Lukashenko allowed Moscow to use Belarusian territory to launch its full-scale invasion on Ukraine in February 2022 and has since let his country be used as a base for Russian nuclear weapons.

The Belarusian Hajun project, which monitors military activity in the country and which is viewed as an extremist formation by Belarusian authorities, said a large column of at least 60 vehicles entered Belarus overnight Friday from Russia.

It said the vehicles, including trucks, pickups, vans and buses, had licence plates of the self-styled Donetsk and Luhansk people's republics in what is internationally recognised as eastern Ukraine. In a move widely condemned as illegal, Moscow moved last year to annex the republics, which have been Russian proxies since 2014.

Hajun said it appeared that a Wagner column was headed to Tsel in central Belarus, where foreign reporters were last week shown a camp with hundreds of empty tents.

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