Belarus, Russia’s closest ally, has released an American prisoner and two others from jail, an exiled opposition group said on Wednesday, in the latest sign that the autocratic Belarusian president, Aleksandr G. Lukashenko, was looking for ways to improve frozen relations with the West.

The releases, announced by an opposition group led by Svetlana Tikhanovskaya in Lithuania, a neighbor of Belarus, followed what Western diplomats said was a secret visit on Wednesday to Minsk, the Belarusian capital, by a U.S. deputy assistant secretary of state, Christopher W. Smith. The group did not identify the American who was freed.

The State Department did not respond to messages seeking comment on whether Mr. Smith had traveled to Minsk, in what would be the highest-level visit to Belarus by an American official since Mike Pompeo, a secretary of state during President Trump’s first administration, went there in 2020 seeking to “normalize” ties.

Mr. Smith, a holdover from the Biden administration, last month helped secure the release of another American citizen held in Belarus, Anastassia Nuhfer.

Franak Viacorka, chief of staff to Ms. Tikhanovskaya, said in a video posted on Telegram that he had visited the U.S. Embassy in Vilnius, Lithuania, and picked up one of the people he said had been released, Alena Movshuk, whom he described as an activist.