Kidnappers in Ukraine have released an employee at a United Kingdom-registered cryptocurrency exchange after getting more than $US1 million ($1.3 million) in bitcoins as ransom, an adviser to the Ukrainian interior minister has announced.

Pavel Lerner, a leading analyst and blockchain expert, was abducted by unknown masked people on December 26, according to a statement by his company, EXMO Finance, on its website.

"This is the first such case in Ukraine linked to bitcoins," Anton Gerashchenko, an adviser to Interior Minister Arsen Avakov, said in a phone 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.