North Korean leader Kim Jong Un provides field guidance to the Paektusan Architectural Institute in this undated photo released by North Korea's Korean Central News Agency in Pyongyang Thursday. Reuters

Strange strings of numbers on the radio in North Korea have some authorities in Seoul worried the country may be revisiting a Cold War strategy that let them communicate in code. Twice in the past five weeks, a woman has taken over the state station's airwaves to read out a sequence of numbers, the Associated Press reported. The last broadcast of this nature in the North was in 2000, before the two Koreas held a summit to reunify separated families, according to the Korea JoongAng Daily.

With relations between the nations now breaking down again, listeners heard a woman come on the air early Friday morning and say, "Starting now, I will give review work to No. 27 exploration agents," according to Yonhap. She continued, saying phrases like "on page 459, No. 35," "on page 913, No. 55" and "on page 135, No. 86." It went on for 14 minutes, much longer than a similar two-minute broadcast in late June.

North Korea used to use — and the South and other countries still use — number stations to covertly give information to spies. An undercover agent simply tunes in at a specific time and translates the numbers into words using a code they carry with them.

"It's classic — the safest way to deliver messages, it leaves no trace," a former agent identified only as Yeom told Reuters in 2014.

In the case of the Koreas in the Cold War, the message sender and the spy would often each have a copy of the same book that would serve as a guide for decrypting the numbers, according to Yonhap. That's why the radio host's announcement that the numbers Friday and in June were review assignments in physics and math for "expedition members" caused concern, the AP reported.

But at least one expert pointed out that number stations wouldn't be the most modern or effective way to transmit information. As The Daily Beast wrote earlier this year, "South Korea and North Korea each run numbers stations, presumably to communicate with spies in each other’s countries, though it’s possible that each is trying to convince the other that’s what it’s up to."

Also on Tuesday, South Korea's military announced that Pyongyang had launched three ballistic missiles, likely less as a test and more to show its strength, according to Reuters.