North Korea's state radio has recently broadcast strings of indecipherable numbers in a possible move echoing a Cold War-era method of sending coded messages to spies operating in South Korea.

A female announcer at the radio station read numbers for two minutes on June 24 and 14 minutes on Friday, according to Seoul's Unification Ministry and National Intelligence Service, including phrases such as 'turn to page 459, question 35' in what she described as a mathematics assignment.

During the Cold War, Pyongyang sent such numbers via shortwave radio to give missions to agents dispatched to South Korea, according to captured North Korean spies.

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North Korea's state radio has recently broadcast strings of indecipherable numbers in a possible move echoing a Cold War-era method of sending coded messages to spies operating in South Korea

'Now we'll begin a mathematics review assignment for members of the 27th expeditionary unit of the distance learning university,' the woman's voice crackled over the radio.

'Turn to page 459, question 35, 913, question 55; 135, question 86.'

The messages, a recording of which was broadcast by South Korean TV channel KBS, were disguised as a mathematics lesson for distance learners and reappeared on North Korean radio station Voice of Korea in the early hours of Friday.

The radio messages, also known as numbers stations, work by broadcasting strings of seemingly random numbers over shortwave signals to an agent in the field.

The technique, a method of sending one-way secret messages, dates to the French Resistance in World War Two and is still in use by some governments today.

South Korea jams most North Korean radio frequencies but Pyongyang-based Voice of Korea broadcasts on shortwave signals which can be picked up far beyond the Korean peninsula, and are difficult to jam.

The receiving agent, armed with a radio and a pen, uses an easily concealed pad with corresponding letters on it to listen to and decrypt the secret message.

A female announcer at the radio station read numbers for two minutes on June 24 and 14 minutes on Friday including phrases such as 'Number 35 on Page 459' and 'Number 55 on Page 913', in a mimic of Cold War-era tactics in which shortwave radios like this were used

'(North Korean) numbers broadcasts have been on hold for quite some time but have recently resumed, something we think is very regrettable,' Jeong Joon-hee, a spokesman for South Korea's unification ministry, told a media briefing on Wednesday.

It was not clear whether the signals were meant to deceive or deliver genuine instructions.

'I can't speak to their intentions, but we hope that the North will refrain from an old practice like this and behave in a manner that's conducive to improving South-North ties,' Jeong said.

Neither the Unification Ministry nor the NIS elaborated on whether South Korea believes the North's recent broadcasts were meant to send information to agents in the field.

The North reportedly stopped such broadcasts once it could communicate with its spies overseas via the internet, and as animosities with South Korea eased following a historic inter-Korean summit meeting in 2000.

Relations have deteriorated greatly since then as North Korea has pursued the development of nuclear weapons despite international sanctions.

Some experts in Seoul view the messages as a North Korean attempt to wage psychological warfare.

Yoo Dongryul, head of the Seoul-based Korea Institute of Liberal Democracy, said the North may be trying to deceive South Korean intelligence officials into believing it's moving to increase its espionage operations.

He said it's unlikely the North would rely on old-fashioned 'number stations' broadcasts, whose hard-to-reset coding patterns had already been exposed to South Korean intelligence officers.

He said North Korea currently uses a more sophisticated espionage communication method known as steganography, in which secret messages are hidden within audio and video files.

On Tuesday, North Korea fired three ballistic missiles into the sea, according to Seoul defense officials

For decades after the end of the 1950-53 Korean War, the rival Koreas sent agents across their heavily fortified border to infiltrate to each other's territory.

But in recent years, both sides are believed to be focusing on less risky intelligence-gathering activities, such as information from the internet and satellite photos.

Seoul accuses Pyongyang of sending spies disguised as ordinary refugees seeking to resettle in South Korea or nurturing pro-North figures in the South.

News of the North Korean broadcast came as North Korea is angrily reacting to the planned deployment of an advanced U.S. missile defence system in South Korea.