The National Intelligence Service, or NIS, says Pyongyang "has been implanting malicious codes by sending enticing text messages" from late February to early March, adding that a fifth of those targeted were successfully hacked. The hackers stole text messages and voice communications, and they secured the numbers of senior government officials, it said.
The NIS has not disclosed which officials have been affected. The spy agency also said North Korea hacked a company that provides security software that is used by more than 20 million people for online banking and card payment. The NIS says no one was affected by that attack; only servers were damaged.
But the intelligence agency says it thinks that last year, Pyongyang turned 60,000 computers into "zombies," or computers that have been compromised by hackers and can then be used for cyberattacks. It adds that Pyongyang took control of 10,000 computers in January alone.
North Korea is known to have an extensive cyberarmy. South Korea's Defense Ministry estimates 6,000 people may be part of it. North Korea has not commented on the NIS claims and often denies reports that it has infiltrated South Korean computer systems.
North Korea threatened a "preemptive and offensive nuclear strike" on the United States and South Korea on Monday as joint military drills between the two countries got underway. The annual exercises are to the largest ever, with 300,000 South Korean troops and 17,000 Americans.
Pyongyang called them "the most undisguised nuclear war drills aimed to infringe upon the sovereignty of DPRK," while Washington and Seoul said they were "defensive in nature."
Meanwhile, South Korea is keeping up the pressure on its northern neighbor, announcing new and expanded sanctions Tuesday after Pyongyang's January nuclear test and February satellite launch, widely seen as a long-range missile test.
Lee Suk-joon, minister for government policy coordination, announced that 38 people and 24 organizations from North Korea believed to be connected to the country's weapons of mass destruction program have been blacklisted and will face financial sanctions. A citizen of Singapore and a citizen of Taiwan have also been added to the list, as well as foreign organizations from Egypt, Singapore, Myanmar, the British Virgin Islands, Thailand and Taiwan.
Any foreign ships that have visited North Korea will not be allowed into South Korean ports for 180 days after that visit, Lee said. Ships that have been registered by North Korea in a third country will also be banned from South Korean ports.