Seoul, Jun 22 (IANS): A hacker group has threatened a cyberattack against seven major South Korean banks if they do not pay 360 million won ($315,000) in the virtual currency bitcoin.

The group, called Armada Collective, has threatened to carry out a distributed denial-of-service (DDoS) attack if the demanded money is not paid by June 26, South Korean news agency Yonhap reported on Thursday.

Among those affected are the country's three largest banks - KB Kookmin Bank, Shinhan Bank and Woori Bank - and others, such as KEB Hana and NH Bank, according to South Korea's financial authorities.

The banks have boosted their security systems in the face of a possible attack that could disrupt their websites.

DDoS is a common cyberattack tactic that involves trying to overwhelm a website or service with a large volume of traffic from diverse sources, reports Efe news.

The Armada Collective has also threatened other companies in the past with such attacks, demanding payments in the bitcoin cryptocurrency.

In November 2015, several secure email services were targeted by the group, which has also made cyberattack threats in countries including Switzerland and Thailand.