Hackers have stolen cryptocurrency worth $625 million from a digital ledger used by players of the popular block-chain based online game Axie Infinity in the biggest digital cash thefts ever.

The team at Sky Mavis, maker of battle and trading game Axie Infinity, discovered the security breach on Tuesday after a user was unable to withdraw ether, according to the company.

Ronin Network said the attack targeting its blockchain netted 173,600 ether worth about $600 million and $25.5 million worth of stablecoin, a digital asset pegged to the US dollar.

"Most of the hacked funds are still in the hacker's wallet," Ronin said in a post revealing the theft.

Ronin was still investigating the hack, but said that hackers got hold of private "keys" to withdraw digital funds.

"We are working with law enforcement officials, forensic cryptographers, and our investors to make sure there is no loss of user funds” Ronin said.

Axie Infinity is an online game started in 2018 in which people collect and create mythical creatures and then battle them against other users. It uses non-fungible tokens to track ownership and reward users for playing. It is run by Sky Mavis, a Vietnam-based game developer that has attracted high-profile investment from the venture capital firm Andreessen Horowitz and Dallas Mavericks owner Mark Cuban.

The Philippines accounts for about 35 percent of Axie Infinity traffic  and the biggest share of its 2.5 million daily active users. The country’s good command of the English language, strong gaming culture and smartphone usage have contributed to the game’s soaring popularity, according to Sky Mavis.

Also read: Away from the battlefield in Ukraine, cyber warfare heats up between Russia and US