North Korean submarines have been removed from the public list of the International Maritime Organization only a day after they were listed, with South Korea’s Joint Chiefs of Staff looking into circumstances surrounding the curious move.

The South’s JCS said Friday that the previously listed North Korean submarines could no longer be found on the IMO’s Global Integrated Shipping Information System.

“North Korea has withdrawn its registration (of the submarines), and the motivations behind the decision remain to be analyzed,” JCS spokesperson Col. Lee Sung-jun told reporters.

“Our JCS and the Navy are closely monitoring the relevant developments.”

The JCS believes the submarines were taken off the IMO list at the request of North Korea.

While North Korea had registered some military vessels with the IMO in the past, this is the first instance of its submarines being put on the list.