South Korea’s spy agency said Tuesday that North Korea had hacked into smartphones belonging to a number of key government officials, intensifying cyber-attacks after conducting its fourth nuclear test.

South Korea's spy agency said Tuesday that North Korea had hacked into smartphones belonging to a number of key government officials, intensifying cyber-attacks after conducting its fourth nuclear test.

The revelations by the National Intelligence Service (NIS) came as the government pushes through parliament an anti-cyber terrorism law that critics say would grant the agency unmatched surveillance powers over cyberspace, including messenger servicing networks.

In a statement, the NIS said the North stole phone numbers and texts from the smartphones of dozens of key South Korean officials between late February and early March.

It also attacked the server of a major software firm specializing in providing security software for Internet banking.

"North Korea has been mounting a series of attacks against our cyberspace" following its fourth nuclear test on January 6, the statement said, adding that they appeared to have been preparation for a major cyber assault on South Korea's banking network.

"If left unchecked, it would have resulted in major financial chaos, such as paralysis of Internet banking systems and unwanted transfers of deposits", it said.

Seoul has blamed North Korean hackers for a series of past cyber-attacks on military institutions, banks, government agencies, TV broadcasters and media websites as well as a nuclear power plant.

The United States also said the North was behind a damaging cyber-attack on Sony's Hollywood film unit over its controversial North Korea-themed satirical film "The Interview" in 2014.