Belarus said Thursday it had opened a criminal probe into planned acts of terror by Russian mercenaries held ahead of presidential polls, adding that it was tracking down dozens more.
The ex-Soviet country's security services on Wednesday arrested a group of 32 Russian fighters as well as one other man in a different location.
Belarus security council chief Andrei Ravkov said a criminal probe had been launched and that the men face charges of preparing "terrorist acts".
"Thirty-three have been detained, there are up to 200 or thereabouts on the territory (of Belarus)," Ravkov said.
He said "a search is going on" to find the others, complaining that it was "like looking for needles in a haystack."
Belarus's KGB security service said the detained men were members of the Wagner group, a shadowy private military firm reportedly controlled by an ally of Russian President Vladimir Putin which promotes Moscow's interests in Ukraine, Syria and Libya.
The arrests came ahead of presidential polls on August 9 in which strongman Alexander Lukashenko is seeking a sixth term.
Lukashenko has detained would-be opposition candidates and their supporters.
Protests have erupted across the ex-Soviet country of 9.5 million people, with a 37-year-old woman political novice, Svetlana Tikhanovskaya, quickly emerging as Lukashenko's main rival.
Lukashenko has accused some of his critics of being controlled by "puppeteers" in Moscow.
Ravkov spoke after meeting the candidates standing against Lukashenko, including Tikhanovskaya, and warning them that security measures would be stepped up at rallies.
State news agency Belta said the authorities had received information about the arrival of 200 fighters in Belarus "to destabilise the situation during the election campaign".
Lukashenko at an emergency meeting of his security council on Wednesday demanded an explanation from Moscow.
"If they are guilty, it's necessary to emerge out of this situation with dignity," Lukashenko said in televised remarks.
Russian Ambassador to Belarus Dmitry Mezentsev said Thursday that he had been invited to the Belarusian foreign ministry that morning to discuss the case, Russian news agencies reported.
Russia is Minsk's closest political and economic ally but relations have been strained.
The Belta news agency said the detained Russians sported "military-style clothing" and carried heavy cases.
It also reported that the alleged militants gave themselves away because unlike ordinary Russian tourists, they did not drink.
"They did not consume alcohol or visit entertainment venues. They kept to themselves in order not to attract attention," Belta said, adding that the men stayed at one of the country's spas.
National television showed several Russian passports that allegedly belong to the detained men, as well as stacks of dollar bills, packets of condoms and pieces of paper with Arabic script.
The men appeared to also have Sudanese pounds in their possession.
Russian author Zakhar Prilepin, who fought alongside Moscow-backed separatists in eastern Ukraine, said he knew some of the detained men.
Writing on Facebook, Prilepin suggested that the fighters probably used Belarus as a transit point and were en route to "some other destination," which he said Belarus "surely knows very well".
Unlike Russia, Belarus has not enforced a lockdown due to the coronavirus pandemic and operates flights as usual.