The detentions come a week after Belarusian security services detained a group of Russians who the authorities said were plotting violent protests with members of the opposition.
Belarus security forces have arrested a number of US national ahead of a presidential election scheduled to take place on Sunday.
President Alexander Lukashenko announced the arrests on Wednesday, giving no details on when or why they have been detained.
Lukashenko is facing the biggest challenge in years to his long rule and has launched a crackdown on opposition protesters whom he accuses of plotting with foreign backers to overthrow him.
Belarus has sought to mend fences with the Washington as ties fray with traditional ally Moscow, and in February hosted Secretary of State Mike Pompeo, the most senior US official to visit in more than two decades.
"Some people were detained with American passports, married to Americans, working in the State Department," the Belta news agency quoted Lukashenko as saying.
The US embassy in the capital Minsk did not immediately reply to a request for comment.
#Belarus.Between May 6 and August 5, at least 1362 people were arbitrarily detained,@viasna96 reports.There are 25 political prisoners.Independent observers are prevented from doing their job and detained at polling stations.Dozens of foreign journalists are waiting for accredit. pic.twitter.com/FXeHSRFPfw— Hanna Liubakova (@HannaLiubakova) August 6, 2020
Meddling in elections?
Lukashenko has accused Moscow and Western countries of meddling in the elections and last week, security services detained a group of Russians who the authorities said were plotting violent protests with members of the opposition.
Earlier on Thursday, Belarus and Russia had traded barbs again over the suspected Russian mercenaries .
Russia has said the men were employees of a private security firm and were passing through Belarus on their way to Latin America.
"The guilt of the detained citizens in Belarus is not backed up by any evidence. They must return to Russia," Russian foreign ministry spokeswoman Maria Zakharova was quoted by the Interfax news agency as saying.
A 65-year-old former Soviet collective farm manager, Lukashenko, has brooked little opposition to his rule since he rose to power in 1994. But he faces a wave of dissent over his handling of the coronavirus pandemic and the economy, and his human rights record.
More than 1,300 people have been detained in a widening crackdown, rights groups say.
Lukashenko's main challenger is Svetlana Tikhanouskaya, who decided to run for president after her husband, who had hoped to be a candidate, was arrested in May.
A former English teacher, Tikhanouskaya plans a mass rally on Thursday evening despite a ban by the authorities. Her campaign chief was released after being briefly detained.
Belarusian air and anti-aircraft forces started military drills on Wednesday ahead of Sunday's election.
Military jets exercised in taking off from a highway strip in Brest region, striking ground targets and landing.
Opposition candidate's aide briefly detained
The campaign manager of Belarus's leading opposition presidential candidate was briefly detained on Thursday, just days before the vote, her office said.
A spokeswoman for presidential hopeful Svetlana Tikhanovskaya said that Maria Moroz was detained by the interior ministry after visiting the Lithuanian embassy in Minsk, then quickly announced she had been freed.
Tikhanovskaya has drawn huge crowds to campaign rallies throughout the country ahead of the vote after she was allowed to participate in place of her husband who was jailed and barred from running.
Yet another friend, Leonid #Kulakov, arrested early morning at home in #Belarus. He seldom went out nowadays because of poor health - high blood pressure. The arrest itself is a torture for him and the thugs will add more in prison. #EU is still to shy to sanction torturers pic.twitter.com/wdJfpkXe24— Andrei Sannikov (@A_Sannikov) August 6, 2020