Hacked cryptocurrency exchange Coincheck has pledged that most of the victims of a recent $500m cyberattack will be repaid - but regulatory officials are yet to be convinced.

In what has been described as one of the biggest cryptocurrency heists of all time, the cyberattack on the Tokyo-based company left 260,000 investors out of pocket. The hackers stole 58 billion yen ($530m, £380m) in a virtual currency called NEM coin.

A statement on Sunday (28 January) said Coincheck officials would return 46.3 billion yen to customers – roughly 90% of the sum stolen.

But a timescale and method for repayment has not been revealed.

According to the Financial Times (paywall), Japan's Financial Services Agency is not convinced that Coincheck has enough money in its reserves to cover the losses.

The agency has requested up-to-date balance sheets from the company to glean better insight into its finances.

The FSA has ordered the exchange to respond to a list of questions by 13 February, it has emerged. Reports suggested that it may soon conduct an on-site inspection.

A letter to Coincheck demanded information on how it would respond customers, strengthen security, establish a new system risk management system and formulate measures to "prevent a recurrence" of the hack.

"We apologise for any inconvenience and worry to you and everyone involved, including our customers," Coincheck said. "By steadily implementing improvement measures to be formulated in the future, we will make our best efforts to restore customer trust."

Tokyo's Metropolitan Police Department will launch an investigation into the theft, according to the Kyodo News outlet. It said that stolen funds had not yet been converted into cash.

Coincheck said on Friday (26 January) that the targeted NEM coins were stored in a "hot wallet", connected to the internet. Hackers reportedly accessed the private key for the wallet where all the NEM coins were stored.

The exchange was forced to halt all withdrawals.

The hack is on the same level as the 2014 attack on another Tokyo exchange, known as Mt. Gox, which went bankrupt after cybercriminals stole about half a billion dollars in bitcoin.

Coincheck president Koichiro Wada (L) bows in apology for the hack AFP/Getty Images