Legal action has been launched following one of the world's largest cryptocurrency hacks.

The Japanese virtual currency trading hub Coincheck said on its website in January that hackers had stolen a reported 58 billion yen ($660 million) worth of the cryptocurrency NEM.

Coincheck says it plans to return about 46.3 billion yen ($523 million) to investors but has not announced the timing and method for the reimbursal.

The company has allowed customers to convert their NEM to yen, but it has not allowed them to move their digital currencies into their own wallets.

Lawyers have launched proceedings against the company in the Tokyo District Court to fight the restriction on withdrawing cryptocurrencies from the exchange.

“Coincheck says it's a security issue which is preventing them from repaying customers their cryptocurrency,” lawyer Jun Tabata said.

“But we are saying if you've got the currency available you must provide it to us because that's what your rules state.”

Experts speculate that Coincheck simply does not have NEM left to give back to customers.

The foundation that created the NEM cryptocurrency has added a tagging system to flag the stolen currency if the hackers start moving it.

It should be able to flag the funds if the criminals try to cash them out or use them for other cryptocurrencies.

Investigations into who is behind the theft are ongoing.