South Korea's spy agency said North Korean hackers were behind attacks on cryptocurrency exchanges this year in which some 7.6 billion won ($6.99 million) worth of cryptocurrencies were stolen.

Citing, the South Korean National Intelligence Service (NIS), the Chosun Ilbo newspaper reported on Friday that the cyber attacks attributed to North Korean hackers also included the leaking of personal information from 36,000 accounts from the world's busiest cryptocurrency exchange Bitthumb in June.

Attacks also included the theft of cryptocurrencies from accounts at exchanges Yapizon, now called Youbit, and Coinis in April and September, it said. The paper also cited the NIS as saying North Korean hackers had also demanded 6 billion won ($5.5 million) from Bitthumb in return for deleting the leaked personal information.

The NIS found that the malware used in hacking the exchanges was made with the same method as malware used in hacking Sony Pictures and the central bank of Bangladesh in 2014 and 2016 respectively, the Chosun Ilbo reported.

The NIS also said emails used in the attacks used North Korean internet addresses, according to the Chosun Ilbo.