North Korea has accused US and South Korean authorities of fabricating the results of a probe that concluded Pyongyang sent small surveillance drones to spy on key South Korean installations.

A spokesman for the North's military attacked the US for what it said was a blindly-backed confrontational conspiracy devised by the government of South Korean president Park Geun-hye, whom it called a "political prostitute".

"If Washington pays heed only to what its stooges trumpet, it is bound to be accused of being a senile grandfather trying to stop a child from crying," the unnamed spokesman said in a statement carried by the official KCNA news agency.

South Korean ministry of defence spokesman Kim Min-seok said the North's statement was "deeply regrettable".

"North Korea isn't a real country is it? It doesn't have human rights or freedom. It exists solely to prop up a single person," Mr Kim said at a briefing in Seoul.

South Korean and US officials jointly examined three drones that were recovered in three different locations near the Korean border over a two-week period starting in late March.

The second was discovered soon after a three-hour artillery barrage between North and South Korea in waters near a disputed maritime border.

Seoul said recovered data showed they had been pre-programmed to fly over South Korean military installations and then return to the North.

In April, North Korea proposed a joint probe into the crashed drones with the South, but Seoul rejected the proposal.

North Korea said in the statement the joint investigation into the origin of the drones was a "charade", designed to divert public criticism of the South Korean government's handling of the Sewol ferry tragedy.

Ms Park's government has faced continued criticism for its handling of the disaster from the families of the ferry victims, many of whom believe a swifter initial response could have saved many more lives.

The two Koreas have stepped up their rhetoric in recent weeks, amid signs that North Korea is preparing to conduct a fourth nuclear test.

North and South Korea are technically still at war after their 1950-53 conflict ended in a truce, not a peace treaty.

The North Korean statement was the latest in a recent series of written attacks by the North against the South Korean president, whom it has previously likened to a "comfort woman".

In April, North Korea described Barack Obama as Ms Park's "pimp", and in an article this month called the US president a "wicked black monkey".

AFP/Reuters