US Secretary of State Mike Pompeo spoke by phone with Belarus President Alexander Lukashenko on Saturday to call for the release and departure from the country of a high-profile American political strategist, the State Department said.
Vitali Shkliarov, a Harvard University fellow who has advised presidential candidates in the US, Russia and Ukraine, was detained on July 29 ahead of August presidential elections that Lukashenko claims to have won but which the opposition says were rigged.
His lawyer said on Monday that he had been moved to house arrest.
"The secretary called for the full release and immediate departure from Belarus of wrongfully detained US citizen Vitali Shkliarov and reaffirmed US support for the democratic aspirations of the people of Belarus," a State Department spokesperson said.
Shkliarov, born in the Belarusian city of Gomel, worked on the presidential campaigns of both Barack Obama and Bernie Sanders.
He has been charged with organizing actions violating public order.
The election was followed by a crackdown on the opposition, with all main challengers to Lukashenko either being arrested or leaving the country.
Protests against Lukashenko have persisted, despite a police threat to open fire on demonstrators.
US ambassador to the Organization for Security and Cooperation in Europe (OSCE) James Gilmore had earlier this month called for Shkliarov's immediate release and alleged that he had been subjected to "extreme psychological pressure and physical abuse".
His wife Heather Shkliarov, an American diplomat preparing for a posting to Kiev, had also expressed grave concern about his health in a statement quoted by US media in September.
Shkliarov had been actively commenting on the situation in Belarus ahead of the election. Writing in the Russian version of Forbes in July, days before his arrest, he said Lukashenko's election campaign was based on "populism and crude force".