The radio broadcast, which seemed to be a book cipher, lasted for 12 minutes, and began at 00:45 a.m. on Friday with a female announcer saying, "Starting now, I will give an assignment to exploration agent No. 27."

The announcer then said, "On page 459, number 35, on page 913, number 55, on page 135, number 86, on page 257, number 2," followed by more numbers.

South Korean intelligence authorities are reportedly trying to figure out why Pyongyang resumed this type of communication, particularly in the digital era when it could have simply given out orders via the Internet.

The revelation has put the Seoul government on alert over possible provocations that can be committed by the North's agents living in the South.

The North halted all such broadcasting after the first inter-Korean summit in June 2000.

The encrypted number messaging, a relic of the Cold War espionage age, requires the sender and the receiver to have access to the same book or reference materials so orders can be handed down by mentioning a page and the position of a word on the page. (Yonhap)