Kidnappers in Ukraine have released an employee at a UK-registered cryptocurrency exchange after getting more than $1m (£750,000) in bitcoin as a ransom, an adviser to the Ukrainian interior minister told Reuters on Friday.

Pavel Lerner, a leading analyst and blockchain expert, was seized by masked abductors on Boxing Day, according to a statement by his company, EXMO Finance, on its website.

“This is the first such case in Ukraine linked to bitcoin,” Anton Gerashchenko, an adviser to interior minister Arsen Avakov, said in a text message.

It was unclear who paid the ransom. Lerner’s work at EXMO did not involve access to the financial assets of its users, the company said, adding that the platform was operating normally.

“At the moment he is safe, and there was no physical harm inflicted on him,” the statement said. “Nevertheless, Pavel is currently in a state of major stress. Therefore, he will not provide any official comments in the coming days.”

News of the release came as bitcoin and other cryptocurrencies rebounded after two days of losses partly related to regulators toughening rules on digital currencies in an effort to curb excessive speculation. Many digital currencies surged in value this year.

Strana.ua, a local news website, had earlier reported that six gunmen wearing dark clothing and balaclavas had snatched Lerner and pushed him into a minibus with stolen number plates.

Kiev police have begun a criminal investigation after a man was kidnapped in the Obolon district, a police spokeswoman said by phone without revealing the name of the victim.

“We would like to note that the story of Pavel’s abduction has overgrown with rumours that might tamper with the official investigation,” EXMO said in its statement.

Separately, the company – which has 900,000 users according to its website – announced on Thursday that it had been hit by a denial-of-service attack.