The two Koreas engaged in an escalating war of words Monday, with Seoul's defence ministry spokesman suggesting it would be best if North Korea simply "vanished".

The heated exchange over the origin of some spy drones that crashed in South Korea fuelled tensions at a time when Pyongyang is believed to be preparing a nuclear test.

South Korea's defence ministry said last week that an investigation into three crashed drones had provided "smoking gun" proof that they were all flown from North Korea.

Pyongyang's denials drew an unusually outspoken response Monday from ministry spokesman Kim Min-Seok.

Arguing that North Korea barely deserved to be regarded as a proper country, Kim said it existed purely to serve the pleasure of leader Kim Jong-Un.

He went on to suggest -- in a remark that is likely to draw a heated response from Pyongyang -- that it would be best if North Korea simply "vanished as soon as possible".

South Korea usually cedes the rhetorical turf to the North when it comes to verbal threats, but Kim's comments suggested its patience might be wearing thin after a recent series of vicious personal attacks from Pyongyang.

As well as repeated sexist swipes at South Korean President Park Geun-Hye, it recently launched a racist diatribe aimed at US President Barack Obama.

In a statement earlier Monday carried by the North's official KCNA news agency, a military spokesman called the findings of the South's probe into the crashed drones a provocative "charade for confrontation".

Labelling South Korean President Park Geun-Hye a "disgraceful political prostitute", the unidentified spokesman accused Seoul of trying to use the drone "conspiracy" to whip up anti-North sentiment with its military protector, the United States.

"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," he said.

The drones were recovered in three different locations in the South near the inter-Korean land and sea borders between March 24 and April 6.

One crashed due to an engine problem, while the other two ran out of fuel.

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

The North has said it would be willing to conduct a joint probe into their origin -- an offer Kim Min-Seok called "as absurd as a criminal demanding to investigate his or her own crime".

Satellite images have shown increased activity in recent weeks at the North's main nuclear test site, although some experts have cautioned that Pyongyang may be looking to rattle nerves by simulating preparations for a detonation.

North Korea has carried out three nuclear tests in 2006, 2009 and 2013.