Story highlights Pyongyang "has been implanting malicious codes by sending enticing text messages," the National Intelligence Service says

It says hackers stole text messages and voice communications, and they secured senior government officials' numbers

South Korea announces new sanctions after Pyongyang's January nuclear test and February satellite launch

(CNN) South Korea's spy agency says North Korea has hacked dozens of top government officials' smartphones over the past couple of weeks.

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.