The Ukrainian border guard service confirmed that Wagner mercenaries had crossed from Russia into Belarus.
An unconfirmed report said that a convoy of about 60 Wagner vehicles crossed the border into Belarus early on Saturday.
Earlier in the day, Belaruski Hajun, a Telegram channel linked to a prominent Belarusian opposition blogger, reported that a large convoy of Wagner vehicles had entered Belarus from Russia.
The vehicles, including vans, trucks and buses, were escorted by Belarusian traffic police on their way to the town of Osipovichy, about 85 km (53 miles) southeast of the capital Minsk, the Belaruski Hajun channel said.
The Belarusian Defense Ministry said the mercenaries and the country's own armed forces plan to hold joint military exercises. Mercenaries will reportedly train Belarusian soldiers.
In June, Wagner chief Yevgeny Prigozhin led a rebellion against Russian President Vladimir Putin. After capturing the city of Rostov, the mercenaries marched on Moscow and stopped 200 km from the capital. The 24-hour rebellion ended.
According to the agreement, the Wagner fighters were given two options. They could join the regular Russian army if they wished, or they could go to Belarus, a close ally of Russia. Belarusian President Alexander Lukashenko offered the Wagner leader to stay in Belarus. However, Prigozhin's current whereabouts are unknown.