Belarus is cracking down on its border with Russia as it searches for 170 mercenaries it says are from the Kremlin-linked Wagner Group military contractor.

Belarussian authorities on Wednesday detained 33 people it said were Russian mercenaries sent to “destabilize” the country ahead of its election. They had received information about the arrival of about 200 fighters in Belarus in total.

Belarus will “significantly strengthen” its security presence at its borders, “including the border with Russia,” security council chief Andrei Ravkov told reporters after an emergency government meeting Wednesday. This will include tracking individuals who cross the state border, including on the land border.

Ravkov on Thursday said “a search is going on” to find the remaining fighters, complaining that it was “like looking for needles in a haystack.”

Belarussian journalists who drove to the land border with Russia reported seeing no expanded military or police presence there as of Thursday.

Russia has denied that the detained men were sent to destabilize Belarus. Its ambassador to Minsk said the men were in Belarus on their way to a third country and had stayed in a hotel after missing their flight.

The high-profile arrests come as Lukashenko, who has been in power for nearly three decades, is seeking a sixth term in the Aug. 9 election despite rising anger against his authoritarian rule.

Belarus and Russia have had virtually no border control since 1995. Russia’s western neighbor has been one of the only countries to keep its borders open during the coronavirus crisis, and Russians have used Belarus as a springboard to travel abroad after their own country grounded international flights.