North Korea's state radio station on Friday resumed the broadcast of mysterious numbers after a two-week hiatus, which some experts say are coded messages to its agents in South Korea.

A female announcer at Radio Pyongyang read numbers for more than four minutes starting at 12:45 a.m., calling out numbers, such as "No. 68 on Page 509."

The announcer said she is "giving review works in basic information technology lessons of the remote education university for No. 27 expedition agents."

Since June, North Korea has now carried out encrypted numbers broadcasts five times.

Broadcasts of mysterious numbers were 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.

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

Experts are divided over whether North Korea is sending instructions for espionage missions in its latest broadcasts. Some analysts said the move is a deceptive strategy aimed at sparking tension within South Korea.

Tensions are already running high on the divided peninsula as North Korea fired off a submarine-launched ballistic missile on Wednesday in protest against an annual joint military drill between Seoul and Washington.

Relations between the two Koreas are at one of their lowest points since North Korea conducted its fourth nuclear test in January and launched a long-range rocket in the following month. (Yonhap)