South Korea is scrambling to protect its computer systems after attacks against government agencies and financial institutions exposed vulnerabilities in a country that has the deepest Internet penetration anywhere.

A large government task force is working on a cyber security master plan to address the threats, and officials said they would allocate fresh budgets and personnel to boost protection of national economic and industrial installations.

Governments, multinational corporations and global institutions are facing a rash of attacks on their computer systems.

The U.S. Senate's website was hacked over the weekend, a major embarrassment although hackers were not able to reach behind a firewall into a more sensitive part of the network.

South Korea, still technically at war with North Korea, is especially vulnerable due to high Internet penetration and as the obvious likely target of the reclusive rival, officials said.

Ensuring cyber security is no longer a matter of choice but is an issue of top priority that impacts national security, an official at the nation's communications watchdog said. The official asked for anonymity because he was not authorized to speak to the media.

Several government agencies including the presidential Blue House and the national spy agency saw their computer networks slowed or crippled for several hours in an attack in 2009 where malicious codes flooded the network with fake access requests.

A similar attack recurred in March, although the impact was limited after government and private online security experts tracked and disabled the software.

Nonghyup, a large commercial bank, suffered a massive network failure in April that affected millions of users, exposing vulnerabilities of the thousands of financial institutions and utilities that rely heavily on networks to keep Asia's fourth largest economy rolling.

More than 95 percent of the country's households have permanent access to the Internet, thanks to heavy government investment in networks.

South Korean prosecutors said North Korean computer hackers were responsible for bringing down the network of the Nonghyup, calling it an unprecedented act of cyber terror. The bank's credit card authorization and ATM networks were disrupted and all electronic transfers disabled for more than a week.

North Korea subsequently denied the charge and accused Seoul of inventing a conspiracy to fuel confrontation, although a North Korean defector warned recently that Pyongyang was recruiting thousands to its cyber warfare unit.

Lee Ho-woong, the head of Security Response Center at Ahnlab, a cyber security firm based in Seoul, said the attacks were getting complex.

Advanced persistent threat that uses a combination of various IT technologies in a systematic manner for economic and political purposes has been a major and continuing trend for some years now, he said.

Since the attacks, government agencies had taken steps to boost readiness. The central bank had increased computer security measures after the attack on Nonghyup bank, a bank technology official said.

The finance ministry had launched a Cyber Security Center for 24-hour vigil of the government's finance and economic agencies, Kang Cheol-won, head of security at Bank of Korea, said.

(Additional reporting by Hyunjoo Jin, Miyoung Kim and Kim Yeonhee; Editing by David Chance)