The U.S. Embassy accused Belarus on Wednesday of politically motivated imprisonment of two Belarusian opposition leaders and called for their immediate release.
Opposition groups are preparing for a rally Friday, which has been banned by authorities.
Seven other activists were also detained this week by the Belarusian security agency, which still goes under its Soviet name, the KGB. They are awaiting trial.
"We call for an end to political harassment by the authorities and the immediate release of Sevyarynets and Shein, and all other political prisoners in Belarus, including former presidential candidate Alexander Kozulin," the U.S. Embassy said in a statement.
The Belarusian authorities did not respond to the U.S. criticism.
Belarus has been ruled since 1994 by Alexander Lukashenko, an authoritarian president who has quashed dissent and closed down opposition media. The United States and the European Union have imposed sanctions on him and top members of his government.