SEOUL: Computer networks at major South Korean banks and top TV broadcasters crashed simultaneously yesterday, paralyzing bank machines across the country and prompting speculation of a cyber attack by North Korea.

Screens went blank at 2 p.m. (0500 GMT), the state-run Korea Information Security Agency said, and more than seven hours later some systems were still down.

Police and S. Korean officials couldn’t immediately determine responsibility and N. Korea’s state media made no immediate comments on the shutdown. But some experts suspected a cyber attack orchestrated by Pyongyang. The rivals have exchanged threats amid joint US-South Korean military drills and in the wake of UN sanctions meant to punish N. Korea over its nuclear test last month.

The network paralysis took place just days after N. Korea accused S. Korea and the US of staging a cyber attack that shut down its websites for two days last week. Loxley Pacific, the Thailand-based Internet service provider, confirmed the N. Korean outage but did not say what caused it.

The S. Korean shutdown did not affect government agencies or potential targets such as power plants or transportation systems, and there were no immediate reports that bank customers’ records were compromised, but the disruption froze part of the country’s commerce.

An official at the Korea Communications Commission said investigators speculate that malicious code was spread from company servers that send automatic updates of security software and virus patches.

LG Uplus Corp., which provides network services for the companies that suffered outages, saw no signs of a cyber attack on its networks, company spokesman Lee Jung-hwan said.

The S. Korean military raised its cyber attack readiness level but saw no signs of cyber attacks on its networks, the Defense Ministry said.

No government computers were affected, officials said. President Park Geun-hye called for quick efforts to get systems back online, according to her spokeswoman, Kim Haing.

The shutdown raised worries about the overall vulnerability to attacks in S. Korea, a world leader in broadband and mobile Internet access. Previous hacking attacks at private companies compromised millions of people’s personal data. Past malware attacks also disabled access to government agency websites and destroyed files in personal computers.

Seoul believes N. Korea runs an Internet warfare unit aimed at hacking US and S. Korean government and military networks to gather information and disrupt service.

In 2011, a top security software firm said N. Korea or its sympathizers likely were responsible for a cyber attack against S. Korean government and banking websites earlier that year.

The analysis also said N. Korea appeared to be linked to a 2009 massive computer-based attack that brought down US government Internet sites. Pyongyang denied involvement.

The shutdown comes amid rising rhetoric and threats of attack from Pyongyang over the UN sanctions.