Officials in Seoul described the resumption of the transmissions, disguised as mathematics lessons for long distance students, as "regrettable."

"We urge North Korea [to] desist from such outdated practices," ministry spokesman Jeon Joon Hee told reporters, calling on the North to instead "seek ways to promote inter-Korean ties."

According to the Yonhap news agency, the broadcast lasted 12 minutes from 00.45 a.m. local time on Friday, and was in a form known as a "book cipher."

"Starting now, I will give an assignment to exploration agent No. 27," said a female voice in a section of the transmission, according to Yonhap. "On page 459, number 35, on page 913, number 55, on page 135, number 86, on page 257, number 2."

The broadcasts appeared to be a resumption of North Korea's Cold War method for contacting spies in South Korea, with a list of numbers transmitted via shortwave radio to an agent in the field.

Possible sleeper agents?

The technique, a method of sending one-way secret messages, dates back to the French Resistance in World War Two.

While South Korea jams most radio frequencies from the North, shortwave signals - such as that used by Pyongyang-based Voice of Korea - are particularly difficult to jam.

The last such broadcasts by the North were made in 2000, with the practice stopped after the holding of a first inter-Korean summit.

Yonhap said South Korean intelligence was considering the possibility that such an outmoded method was being used in order to communicate with sleeper agents.

The agency pointed out that the transmissions began again on the same week that Washington finalized a deal to send a US anti-missile battery to South Korea, which could intercept missiles from the North.

North and South Korea are still technically at war following the 1950-53 Korean War which ended in a truce, not a peace treaty.

rc/ipj (dpa, AP)