Popov said that according to the Vienna Convention, the diplomatic presence of countries should be on a parity basis. However, the U.S. employs far more diplomats in Belarus than vice versa.
"We believe this step would be logical. There are no top level visits and economic contacts have been reduced," he said.
"It was not in the interests of Belarus to provoke tension between the two countries. Our actions were in response to the American side's actions," Popov said.
The spokesman said his ministry suggested that the U.S. decide for itself which diplomats should leave Minsk.
Touching upon the issue of visa suspension for Belarusians, Popov said there had not yet been any official notification from U.S. consular services.
The U.S. Embassy in Minsk said in a consular notice posted on its website on Wednesday that it had suspended visa services for Belarusian nationals.
"The U.S. Government is in the process of reviewing the request made by the Belarusian Ministry of Foreign Affairs on March 17 that the U.S. Embassy in Minsk reduce its staffing. Therefore, visa processing has been temporarily suspended while our resources are engaged addressing other priorities," the statement said.
It added that some visa appointments had been postponed.
U.S. Ambassador Karen Stewart left Belarus March 12 after a request from Belarusian authorities.
Almost two weeks ago, Minsk advised Ambassador Stewart to leave the country and recalled its ambassador from Washington for consultations over new sanctions against Belarus' national petrochemical company, Belneftekhim.
Last November, the U.S. introduced sanctions against Belneftekhim and froze the assets of its U.S. subsidiary. Belarus said the moves breached a bilateral trade deal designed to give better access to Belarusian goods and services.
Alexander Lukashenko, the controversial Belarusian leader dubbed "Europe's last dictator" by Washington, pledged last year to retaliate using economic means, and threatened Stewart with expulsion from Minsk.
He also said the U.S. sanctions were caused by Minsk's decision to start oil production work in Venezuela. In December, a joint Belarusian-Venezuelan oil production company was opened in the South American country, with plans to produce about 7 million tons (51.45 million bbl) of oil annually. 40% is to go to Belarus and 60% to Venezuela.
The U.S. and the European Union have accused the Belarusian leader of clamping down on dissent, stifling the media and rigging elections. Lukashenko, who was re-elected to a third term in 2006, is currently barred, along with other senior Belarusian officials, from entering the U.S. and the EU.