South Korean officials have announced that the cyber-attack on the country’s banks and broadcasters came from an internet address in China.
However, the identity of those behind the cyber attack cannot be confirmed.
The telecoms regulator said hackers used a Chinese address to plant a malicious code that hit networks at six organizations on Wednesday.
Officials said they were continuing to investigate the origins of the attack.
North Korea has been blamed for previous attacks in 2009 and 2011.
“Unidentified hackers used a Chinese IP address to contact servers of the six affected organizations and plant the malware which attacked their computers,” said Park Jae-moon of South Korea’s communications regulator.
“At this stage, we’re still making our best efforts to trace the origin of attacks, keeping all kinds of possibilities open,” he said.
Officials stressed that the IP address did not reveal who was behind the attack, as hackers can route their attacks through addresses in other countries to obscure their identities.
However, the discovery has strengthened speculation that North Korea was behind the attack.
Intelligence experts believe that North Korea routinely uses Chinese computer addresses to hide its cyber-attacks.
A taskforce is being formed to analyze the virus and stop further attacks, and free computer vaccines have been handed out to South Korean companies.
Korea’s Communications Commission (KCC) said that the attacks on all six organizations appeared to come from a single entity.
The networks had been attacked by malicious codes, rather than distributed denial-of-service (DDoS) attacks as initially suspected.
Following Wednesday’s attack, the KCC raised its cyber-attack alert levels to “caution,” the third highest out of five levels, news agency Yonhap reported.
Around 32,000 computers were affected by the incident, and some services at Shinhan bank, including internet banking and ATM machines, were disrupted.
However, so far no damage had been detected in public institutions and infrastructure, the KCC was quoted as saying by Yonhap.
The incident comes with tensions between the two Koreas high.
North Korea has stepped up rhetoric in recent days in response to fresh UN sanctions over its nuclear test in February and joint annual military drills between the US and South Korea, which it bitterly opposes.
On March 15, North Korea’s KCNA news agency also accused the US and its allies of “intensive and persistent” hacking attacks on its internet servers.