North Korea's state radio station on Friday resumed the broadcast of mysterious strings of numbers after a two-week silence, which some experts say are coded messages for North Korean spies in South Korea.

A female announcer at Radio Pyongyang read numbers for more than four minutes starting at 12:45 a.m., mentioning the numbers in such a way 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 carried out four separate encrypted numbers broadcasts, including the latest one that took place after regular broadcasting.

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

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

The two Koreas' relations have further worsened since North Korea conducted its fourth nuclear test in January and launched a long-range rocket in the following month.

South Korean authorities are believed to be reviewing measures to brace for the possibility that the broadcasts could prompt North Korean agents to wage terrorist attacks or target high-profiled North Korean defectors. (Yonhap)