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

Radio Pyongyang, started broadcasting messages shortly after midnight (Seoul time), calling out a series of pages and numbers.

The radio announcer "gave review work in metal engineering to No. 21 expedition agents." The content was different from those transmitted in the early hours of Oct. 28.

Since June 24, North Korea has sent out a total of 12 encrypted numbers broadcasts, with three being broadcast in October.

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by using a reference book, although many intelligence officials believe this form of sending orders to be totally outdated.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 missile last month with some observers forecasting another kind of missile provocation in the coming days. (Yonhap)