Japanese police say a group of North Korean hackers likely used a job offer message to steal assets from cryptocurrency exchange DMM Bitcoin in May.

The theft cost the company bitcoin valued at about 308 million US dollars at the time of the attack.

The firm earlier said it is closing its business after handing over the accounts and assets of its customers to another exchange.

Japan's National Police Agency and Tokyo Metropolitan Police Department announced on Tuesday that TraderTraitor was affiliated with the theft.

TraderTraitor is an arm of the Lazarus hacking group, which is believed to be a subordinate organization of North Korean authorities.

The police said an employee of a company that manages cryptocurrency for DMM Bitcoin received a message on social media in March from an individual claiming to be an employment recruiter at another firm.

The message is believed to have planted a computer virus when it was clicked on, allowing the group to steal information about access authority, and to rewrite amounts of transactions and beneficiaries.

The police say the social media account that sent the message and the server it was connected with were related to North Korea.

The Japanese government and the US Federal Bureau of Investigation have issued a joint statement, condemning the hackers and North Korea. They urge people to beware of suspicious messages.