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Its appearance a day after Pyongyang tested a new type of anti-ship missile yesterday, could spark questions about the state of South Korea's air defences at a time when Seoul is trying to rein in the North's nuclear and missile programmes.

In size and shape, the device looked like a North Korean drone found in 2014 on an island near the border, South Korea's Office of Joint Chiefs of Staff said in a statement, adding that authorities plan to conduct a close analysis.

"The drone found this time looks sloppy but slightly more slender than previous ones," a South Korean military official told Reuters on condition of anonymity, because he was not authorised to speak to the media.