By Choi Ha-young

North Korea has hacked one of the online virtual currency trading systems in South Korea, the National Intelligence Service (NIS) claimed Monday, without revealing the name of the system.

Through the cyberattack, tens of billions of won were stolen and transferred to North Korea, the spy agency told the National Assembly Intelligence Committee.

"Last year, a North Korean agency distributed emails to infiltrate the exchanges and customers' accounts. Some exchanges were robbed of virtual currencies worth of tens of billions of won," Democratic Party of Korea Rep. Kim Byung-kee, a committee member, told reporters after the meeting with NIS officials.

The officials said North Korean hackers successfully accessed some private data, but it is uncertain whether each customer was notified about the data leak.

"Capable cyber experts in the NIS are countering such attacks," Kim said.

"The emails were camouflaged as job application letters, playing upon the exchanges' job openings," Kim said.

"While doing so, North Korean hackers incapacitated anti-spyware vaccine programs."

The NIS has urged security experts and North Korea specialists to take care of their email and social media accounts, following the North's continuous cyberattacks, according to Kim. "The NIS said it pre-emptively detected the North's intrusion attempts against a defense industry company and prevented any damage," Kim said.

The committee members declined to reveal the names of the cryptocurrency exchange, citing confidentiality.

Speculation has been rampant about the North's continued attacks on the virtual currency market. Last month, U.S. cybersecurity firm AlienVault claimed it found malware that mines a type of cryptocurrency called Monero. The virtual money, generated by compromised foreign computers, was sent to Kim Il Sung University in Pyongyang, the firm said.