The US accused the republic of "unjustifiably and illegally" detaining the official in violation of the Vienna Convention, which protects the right of diplomats. Last night, the State Department was pondering its next move, which may include a tit-for-tat expulsion of a Belarus diplomat from Washington.
Hostility between the US and Belarus has been growing steadily since November, when the president of the republic, Alexander Lukashenko, 42, a former Soviet collective farm director, forced through a referendum granting himself dictatorial powers.
Matters worsened recently when Belarus refused a visa to an American official from the Soros Foundation, branding him an opposition supporter.
The foundation has supplied millions of dollars for education, ecological and medical programmes in Belarus. This month, the US suspended $40m (pounds 23m) in aid on the grounds that Belarus failed to comply with internationally recognised standards on human rights.
American officials quote a long list of examples, ranging from Mr Lukashenko's referendum, which European monitors concluded was flawed, to censorship, the jailing of opposition politicians and the creation of a puppet constitutional court and parliament.
It is probably no coincidence that the US has highlighted the issue when it is making the case for Nato's expansion into Eastern European nations, such as Poland, Hungary and the Czech Republic.
The expelled US diplomat, Serge Alexandrov, a Belarus national, was one of about 70 people who were arrested on Sunday at an opposition demonstration of 10,000 people, which was broken up by police using clubs and tear gas. Mr Alexandrov, who was a first secretary at the US embassy, was later expelled. Reports on the state-controlled Belarus television claimed he had helped to organise the unauthorised rally and was working as a spy.
A US official said last night that the embassy was satisfied Mr Alexandrov had been carrying out his normal diplomatic duties, adding that the issue was being viewed with the "utmost gravity and seriousness" at the "highest levels" of the State Department in Washington.
"This marks a further step in the movement by the government of Belarus away from democratic reforms and respect for human rights", the official said.
The issue is also awkward for Moscow. Faced with increasingly Westward- looking governments in the Ukraine and the Baltic states, Russia needs Belarus as a corridor through which to pipe its oil and gas to Western Europe. Moves are afoot to further integrate the two countries, which are already bound together in an economic union.
Mr Lukashenko's autocratic conduct, which is generally heavily criticised in the Russian media, complicates Moscow's already uneasy relations with the West. The republic has already been stripped of its "special guest" status in the Council of Europe because its constitution is flagrantly undemocratic. However, yesterday the Kremlin was playing down the fracas, saying it would have no effect on integration and was an internal matter for Belarus.