Belarus has freed 123 prisoners, including prominent opposition activist Maria Kolesnikova, after the US agreed to lift sanctions on the country.
Nobel Peace Prize winner Ales Bialiatski is also among those who have been released following talks in Minsk with US President Donald Trump's special envoy for Belarus, John Coale.
The US has agreed to lift sanctions on potash, a key ingredient in fertiliser and an important export for Belarus, which is a close ally of Russia.
Coale said: "As relations between the two countries normalise, more and more sanctions will be lifted."
Belarus' president, Alexander Lukashenko, is not recognised as president by the EU.
Kolesnikova has been in prison since 2020, much of the time in isolation.
Her sister, Tatiana Khomich, who campaigned tirelessly for her release was able to speak to her by video call soon after and confirmed the news to the BBC.
"She is free, she looks fine, she looks good," said Khomich, who said she was looking forward to giving her sibling a hug.
Kolesnikova was handed over to Ukraine along with 113 other prisoners, according to Kyiv's Coordination Headquarters for the Treatment of Prisoners of War.
In a statement on Telegram, Ukraine said that after receiving the necessary medical assistance, the prisoners will be transported to Poland and Lithuania.
A spokesman for the exiled Belarusian opposition leader Svetlana Tikhanovskaya told Agence France-Presse that the decision to send them to Ukraine was unexpected and had been made by Lukashenko.
Khomich said the first thing Kolesnikova said when they spoke was "thank you to the US administration, President Trump [and] to the Belarus government as well for leading and talking and having these negotiations".
The deal is a major achievement for Lukashenko and the authoritarian leader will also welcome how the Americans have ended his international isolation.
As well as the EU, the US did not recognise Lukashenko as president following unfair elections five years ago which led to mass street protests that were brutally suppressed by police.
Hundreds of people were arrested then – including Kolesnikova - and the intense political repression has continued.
Western sanctions were tightened following Russia's full scale invasion of Ukraine in 2022, when troops entered via Belarus and missiles were launched from its territory.
Coale was quoted by Belarusian state media saying that the sanctions on potash would be lifted immediately,
The US envoy also said he spoke to Lukashenko about Ukraine and what help Minsk could offer in talks with Putin.
The attempt to engage with Minsk is part of a major shift in US policy, which leaves it very much at odds with Europe – where the approach is to sanction and isolate.
Viktor Babaryko, a Belarusian opposition politician who was arrested five years ago, was also released on Saturday along with Marina Zolotova, editor-in-chief of independent news site Tut.by.
