North Korea has resumed the broadcasting of encrypted numbers, a method used in the past to send messages to spies operating in South Korea, a government source said Tuesday.

The source said that propaganda radio station Radio Pyongyang aired a 12-minute shortwave segment last week during which a female announcer read out numbers on what seemed to be from a book. The broadcast started in the early hours of Friday morning and marks the first time Seoul picked up on such communication from the North in 16 years.

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

The encrypted number system, a relic of the Cold War espionage age, requires the sender and the receiver to have access to the same book and other types of reference materials so orders can be handed down by mentioning a page and the position of a word on that. This was a favored method employed by the reclusive country to contact and give orders to spies that infiltrated the South.

The coded radio broadcast began at 00:45 a.m. with a female announcer saying, "starting now, I will give review work to No. 27 exploration agents."

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 scrambling to find 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 timing of the broadcast also is being looked into since it took place just one week after a decision by Seoul to deploy the U.S.-made missile defense system known as the Terminal High Altitude Area Defense system on its soil.

An official of the Korea Institute of Liberal Democracy, a conservative think tank in Seoul, said that in the past, North Korea often used the encrypted number method to hand out orders.

He speculated the resumption of the broadcasting is aimed at intensifying psychological warfare against South Korea as well as escalating tensions on the Korean peninsula. (Yonhap)