The European Union is also considering imposing tougher sanctions over Belarus' disputed post-election crackdown
The United States has called on Belarus to free all of its political prisoners, including prominent opposition figure Andrei Sannikov and his journalist wife Irina Khalip.
State Department spokesman Philip Crowley's call came through a Twitter message after child protection authorities threatened that they would take the couple's 3-year-old son Danil into custody since both his parents were under detention.
Meanwhile, Khalip's mother Lucina said that a local court had allowed her to become Danil's formal guardian after child protection services made a failed attempt to pick him up from kindergarten last week.
Crowley's call came a day after President Alexander Lukashenko was sworn in for a fourth term, a move vehemently criticized by western nations. The government's official mouthpiece, meanwhile, accused the couple of receiving $20 million from donors to topple Lukashenko during Dec. 19 election protests.
"The campaign of repression in Minsk must be reversed, beginning with the immediate and unconditional release of imprisoned journalists and opposition activists," Democratic U.S. Senator Ben Cardin said in statement.
The European Union is also considering imposing tougher sanctions against Lukashenko over Belarus' disputed post-election crackdown.
In a separate development, ambassadors of the European Union distanced themselves from the inauguration of Lukashenko. To show their protest, envoys of Poland, Germany and Lithuania, along with nine other EU nations, left Minsk a day before the swearing-in ceremony. The U.S. and Ukraine also boycotted the ceremony.