North Korea's state radio station resumed broadcasting mysterious numbers Friday after a four-day break that could be some kind of coded message to its agents operating in South Korea.

Radio Pyongyang, the Korea's state-run radio station, started broadcasting messages at 1:15 a.m., calling out a series of pages and numbers before repeating them one more time.

The radio announcer "gave review work in metal engineering to No. 27 expedition agents." The content was different from what has been transmitted this year.

Since June 24, 18 of such encrypted numbers broadcasts have been discovered, with the latest one broadcast Sunday.

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by using a reference book, although many intelligence officials believe this form of sending orders to be totally outdated.

Many have said the broadcast may be some sort of psychological strategy aimed at sparking internal confusion within South Korea.

Pyongyang had initially suspended such broadcasts in 2000, when the two Koreas held their first historic summit.

Tensions are already running high on the divided peninsula after North Korea carried out its fifth nuclear test on Sept. 9 and the unsuccessful launching of two Musudan intermediate-range ballistic missiles last month, with some observers forecasting another kind of missile provocation in the coming days. (Yonhap)