Shinhan Bank and Nonghyup Bank both saw their systems go down briefly at around 2:00pm local time. Shinhan says that ATMs, POS terminals and mobile banking services were all briefly knocked out.

Television networks YTN, MBC and KBS were hit at the same time, with computers crashing and refusing to restart. Reports suggest that skulls popped up on some screens, indicating malicious code, according to the BBC.

A police official told Reuters: "We sent down teams to all affected sites. We are now assessing the situation. This incident is pretty massive and will take a few days to collect evidence."

South Korean officials have refused to comment on who is behind the attacks but speculation has inevitably focused on North Korea, with tensions between the two high in the wake of Pyongyang's recent nuclear test.

Last week the rogue state accused the US and South Korea of carrying out cyber-attacks against it following a two day Internet outage. It also has form, widely assumed to have been behind two sets of attacks - in 2009 and 2011 - on government and business sites in the South.