By Andrius Sytas and Mark Trevelyan

VILNIUS (Reuters) -Siarhei Tsikhanouski, a leading Belarus opposition figure, was freed on Saturday after more than five years in prison, in the most significant move so far by President Alexander Lukashenko to try to ease his isolation from the West.

The 46-year-old was driven across the border into Lithuania for an emotional reunion with his wife Sviatlana Tsikhanouskaya, the leader of the exiled Belarus opposition.

The Lithuanian government said 13 other prisoners were also released and taken there following talks between Lukashenko and U.S. special envoy Keith Kellogg. In total, five Belarusians were freed along with three Poles, two Latvians, two Japanese citizens, one Estonian and one Swede.

Tsikhanouski was seen emerging from a van with a shaven head, smiling and immediately stepping up to hug his wife in a long embrace, a video released by her office showed.

"It's hard to describe the joy in my heart," she said.

Franak Viacorka, Tsikhanouskaya's chief political adviser, told Reuters that Tsikhanouski had lost weight and his health suffered. He said he "looks like a different person", but was in good spirits and had joked with U.S. officials.

Viacorka said Tsikhanouski had described "horrible things that happened to him, but he's not broken... He wants to continue fighting."

In a post on X, Tsikhanouskaya thanked U.S. President Donald Trump, as well as Kellogg and others for their efforts to secure her husband's release.

Lukashenko has been shunned by the West for years after brutally crushing pro-democracy demonstrations in 2020 and then allowing Russian President Vladimir Putin, his close ally, to launch part of his 2022 invasion of Ukraine from Belarusian territory.

In the past year, he has freed more than 300 prisoners in what political analysts see as the start of an attempt to restore relations with Western governments and seek an easing of international sanctions against Belarus. Opposition politician Maria Kalesnikava and Nobel peace prize winner Ales Bialiatski are the most prominent of those who remain behind bars.

"We're not done," Tsikhanouskaya wrote, calling for the release of a further 1,150 prisoners.

TALKS WITH KELLOGG

Lukashenko issued pardons for all those released on Saturday in response to a U.S. request, his spokeswoman Natalya Eismont said. In a statement, she said the Belarusians who were freed had been "convicted of extremist and terrorist activity".

She said the decision to release Tsikhanouski was "taken by the president strictly on humanitarian considerations with the aim of family reunification".

Eismont said Kellogg's talks with Lukashenko had begun on Friday evening and lasted 6-1/2 hours over dinner, covering the Ukraine conflict, the Middle East, relations between Russia, Belarus and China, and sanctions policy.

"It was night when it ended," she said. "The subjects discussed were as current as can be imagined. They talked about things that concern the entire world."

Reuters reported on Tuesday that Kellogg, the highest-ranking U.S. official to visit Belarus in years, saw his mission as one that could help jump-start peace talks aimed at ending Russia's war against Ukraine.

DESTINATION UNKNOWN

Among those released by Belarus was Ihar Karnei, a former journalist at Radio Free Europe/Radio Liberty, RFE/RL President and CEO Stephen Capus said in a statement thanking Trump, U.S. Secretary of State Marco Rubio and others.

Tsikhanouskaya's adviser Viacorka said the freed individuals had no idea where they were going when they were taken from their cells.

"They were handcuffed, brought to cars, brought to the border. They were surprised, they didn't know who released them, why they were released, what was happening. It was so nice to see the happiness in their eyes," he said.

On arrival in Lithuania, they were taken to the U.S. embassy, ate pizza and drank coke, and were given mobile phones so they could speak to their families for the first time in years, he said.

European Commission President Ursula von der Leyen said the release was "fantastic news and a powerful symbol of hope for all the political prisoners suffering under the brutal Lukashenka (sic) regime".

"The free world needs you, Siarhei!", Polish Foreign Minister Radoslaw Sikorski said on X.

Tsikhanouski, a video blogger, was arrested in 2020 when planning to run for president against Lukashenko and convicted the following year of organising mass unrest and inciting social hatred. He was sentenced to 19-1/2 years, one of the longest jail terms in modern Belarusian history.

His supporters said the charges were fabricated and politically motivated. Sviatlana ran in the election in his place, and mass protests broke out when Lukashenko claimed a landslide victory and the opposition and Western governments accused him of rigging the result.

Viacorka said Tsikhanouski's release would spur the exiled opposition.

"It's a big boost to our fight, I think it will attract attention to Belarusian political prisoners and to Belarus in the world right now - we have to use this momentum," he said.

(Reporting by Andrius Sytas in Vilnius and Mark Trevelyan in London; Additional reporting by Fylyp Lebediev and Humeyra Pamuk; Editing by Marguerita Choy)