Belarus said on Thursday it suspected a group of alleged Russian mercenaries it detained the previous day of plotting "acts of terrorism" ahead of a presidential election in August and summoned Russia's ambassador for an explanation.
Minsk's handling of the situation risks worsening relations with traditional ally Russia, which has so far yet to set out its own position.
Belarus said on Wednesday it had detained more than 30 suspected Russian mercenaries near Minsk after receiving information that more than 200 fighters had entered the country to destabilise it ahead of a presidential election on Aug. 9.
Belarusian Security Council State Secretary Andrei Ravkov told reporters on Thursday that up to 200 mercenaries were still in Belarus and that law enforcement agents were looking for them.
State media in Belarus have reported that the detained men worked for Wagner, Russia's best-known private military contractor. The Kremlin, the Russian Foreign Ministry and a business reportedly affiliated with Wagner have not commented. The Russian state denies it uses mercenaries.
Footage of the men's belongings filmed during their arrest showed Sudanese currency and a Sudanese phone card. That prompted some experts to suggest the men may have been transiting via Minsk en route to Africa.
The detentions are the latest twist in a presidential election campaign that has posed the biggest challenge in years to President Alexander Lukashenko's iron-fisted rule of the east European country.
The 65-year-old former collective farm manager is under pressure from critics over what they say is his irresponsible handling of Covid-19, the economy and alleged human rights abuses.
He says he has delivered years of economic and political stability and that the state covers many of people's core needs just as it did in the Soviet era.
Lukashenko has accused opponents of being in cahoots with foreign backers to overthrow him and has jailed two of his main election rivals ahead of the election, which he is expected to win despite his problems.
A senior Belarusian security official has said that 14 of the alleged mercenaries had spent time in the Donbass region of eastern Ukraine, where Ukrainian troops have fought Russian-backed fighters in a conflict since 2014.
Ukraine's Foreign Ministry said Kyiv would consider extraditing the suspected Russian mercenaries.
Lukashenko has ruled Belarus since 1994.