North Korea's state radio station on Friday broadcast a string of mysterious numbers, believed to be a coded message to its agents operating in South Korea.

Radio Pyongyang started to read a series of book pages and numbers about an hour after midnight (Seoul time). The content was the exact same as that broadcast on Nov. 11.

The latest broadcast, the fourth this month, came as Pyongyang is stepping up propaganda against Seoul over a corruption scandal involving President Park Geun-hye and her close confidante. The influence-peddling scandal that broke out in late October has paralyzed the government and sparked calls for Park to step down.

North Korea has sent out a total of 15 encrypted numbers broadcasts since it resumed the Cold War-era means of communication with its overseas secret agents on June 24. The last was conducted on Nov. 20.

Broadcasts of mysterious numbers are considered a kind of book cipher that has been used by North Korea to give missions to spies operating in South Korea. Spies could decode the numbers to get orders by using a reference book, although many intelligence officials believe this form of sending orders to be outdated.

It is also believed to be some sort of psychological warfare aimed at sparking internal discord within South Korea.

Pyongyang had halted such broadcasts in 2000, when the two Koreas held their first historic summit.  (Yonhap)