North Korea threatens to turn South Korea and the US into "a heap of ashes"

North Korea has threatened to turn South Korea and the US into "a heap of ashes through a Korean-style pre-emptive nuclear strike".

The words come as South Korea and the US begin a 12-day military exercise called Ulchi Freedom Guardian, which North Korea has described as an invasion rehearsal.

The exercise involves 25,000 American and 50,000 South Korean troops.

The North Korean Foreign Ministry has described the military exercise as an "unpardonable criminal act" that could bring the peninsula to "the brink of war".

The country often issues such fiery rhetoric when it feels threatened.

China has also voiced concern about the exercise, with the official Xinhua news agency saying it would make North Korea "more aggressive" at an already sensitive time.

On Monday, the UN's nuclear watchdog said North Korea appeared to have resumed activities aimed at making plutonium earlier this year, but added that signs of this had stopped early in July.

On Sunday, the South said its neighbour could resort to assassinations and kidnappings in revenge for recent defections.

An official from South Korea's Unification Ministry said the defection of North Korea's deputy ambassador in London was helping put the North in "a very difficult situation".

"Considering [North Korean leader] Kim Jong-Un's character, it is very dangerous," said the unidentified official.

"It is highly likely that North Korea will make various attempts to prevent further defections and unrest among its people."