• The FBI said hackers linked to North Korea were behind the attack on Axie Infinity's Ronin Network in March. 
• The FBI said the hackers including the infamous Lazarus Group were behind the theft of $620 million.
• The heist is believed to be one of the largest in the crypto world.

North Korea-linked hackers were behind the theft of $620 million in cryptocurrency from players of online game Axie Infinity, US authorities said Thursday.

Axie Infinity is a non-fungible token-based online game that allows players to earn an in-game currency linked to the ethereum blockchain.

Hackers made off with the money after targeting Axie Infinity's blockchain project Ronin Network, which allowed users to transfer crypto in and out of the game.

The video gaming company disclosed the massive security breach last month, and the theft is believed to be one of the largest in the crypto world.

"Through our investigation we were able to confirm Lazarus Group and APT38, cyber actors associated with the DPRK,  are responsible for the theft of $620 million in Ethereum reported on March 29th," the FBI said in a statement Thursday.

The US Treasury Department on Thursday also added a "wallet," or cryptocurrency address, linked to the Axie Infinity hack to its sanction list for the Lazarus Group.

The notorious Lazarus Group has been linked to the WannaCry ransomware attack in 2017 and the hacking of Sony Pictures in 2014. 

During the Sony Pictures attack, hackers demanded the studio withdraw the then-upcoming satirical film "The Interview", which depicted a plot to assassinate North Korean leader Kim Jong Un.

North Korea's hacking program is an important source of revenue for the government to fund its nuclear and ballistic missile activities, according to a United Nations report.

By conducting cyberattacks on cryptocurrency exchanges, Pyongyang avoids international sanctions.

North Korea stole nearly $400 million worth in cryptocurrency in 2021, which made it a "banner year" for the country's cybercriminals, according to blockchain analysis firm Chainalysis.

The country's cyberwarfare unit is believed to number 6,000 hackers, operating from countries including Belarus, China, India, Malaysia and Russia, according to a 2020 US military report.