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Radio Pyongyang sent out secret messages in the early hours of the morning, listing a series of numbers including “No. 69 on page 823”, “No. 92 on page 467” and “No. 100 on page 957”.

The numbers are believed to be encoded with a book cypher – a system common during the Cold War which tells spies to look up specific words on various pages of an agreed text which spell out a phrase.

The numbers are different from those previously sent out by the radio station, which last offered encrypted phrases on Sunday.