North Korea has threatened nuclear attacks on Seoul and Washington as South Korea and the US began annual military drills on Monday.

The North's military has said that it will turn Seoul and Washington into "a heap of ashes through a Korean-style pre-emptive nuclear strike" if they show any signs of aggression toward the North's territory.

The extreme rhetoric from Pyongyang comes at a time of high tension. A US plan to place a high-tech defence missile system in South Korea and the defection of a senior North Korean diplomat, Thae Yong Ho, has deeply antagonised the North Korean leadership.

North Korea weapon tests

Now, state media in Pyongyang have warned the country's "first-strike" units are ready to mount retaliatory attacks on South Korean and US forces involved in the drills.

South Korea's Unification Ministry expressed strong regret over the North's warning, saying the drills with the US are defensive in nature. Seoul and Washington have repeatedly said they have no intention of invading Pyongyang.

This year's manoeuvres, known as Ulchi Freedom Guardian, began on Monday for a 12-day run and are largely computer-simulated war games. The training involves 25,000 American troops and 50,000 South Korean soldiers.

Workers' Party Congress in North Korea Show all 12 1 /12 Workers' Party Congress in North Korea Workers' Party Congress in North Korea North Korean leader Kim Jong Un waves to the crowd as he presides over a mass rally and parade in the capital's main ceremonial square, a day after the ruling party wrapped up its first congress in 36 years by elevating him to party chairman Reuters Workers' Party Congress in North Korea North Korean parade participants wave decorative bouquets of flowers and carry their country's national flag as they march with a model of the Unha pace launch vehicle at the Kim Il Sung Square. Hundreds of thousands of North Koreans celebrated the country's newly completed ruling-party congress with a massive civilian parade featuring floats bearing patriotic slogans and marchers with flags and pompoms AP Workers' Party Congress in North Korea People react as they see North Korean leader Kim Jong Un during a mass rally and parade in the capital's main ceremonial square, a day after the ruling party wrapped up its first congress in 36 years by elevating him to party chairman in Pyongyang Reuters Workers' Party Congress in North Korea High party and military officials react as North Korean leader Kim Jong Un appears at the beginning of a mass rally and parade in the capital's main ceremonial square Workers' Party Congress in North Korea North Korean leader Kim Jong Un is accompanied by high party and military officials as he presides over a mass rally and parade in the capital's main ceremonial square, a day after the ruling party wrapped up its first congress in 36 years by elevating him to party chairman, in Pyongyang, North Korea, May 10, 2016. REUTERS/Damir Sagolj Reuters Workers' Party Congress in North Korea North Korean leader Kim Jong-Un waves from a balcony of the Grand People's Study House following a mass parade marking the end of the 7th Workers Party Congress in Kim Il-Sung Square Getty Images Workers' Party Congress in North Korea Attendees cheer the arrival of North Korean leader Kim Jong-Un during the 7th Workers Party Congress Getty Images Workers' Party Congress in North Korea A hostess and security guard stand inside the April 25 Palace, the venue of the 7th Workers Party Congress Getty Images Workers' Party Congress in North Korea North Korean leader Kim Jong Un attends the first congress of the country's ruling Workers' Party in 36 years Reuters Workers' Party Congress in North Korea Party representatives sit in the hall of the April 25 House of Culture during the party congress in Pyongyang AP Workers' Party Congress in North Korea North Korean leader Kim Jong Un listens during the party congress in Pyongyang. North Korea has brought in more than 100 journalists from around the world to make sure that the 7th Congress of its ruling Workers' Party gets global attention. Four days into the event, they allowed a small number of foreign journalists into the conventional hall where the congress was taking place Workers' Party Congress in North Korea A general view shows the April 25 Palace, the venue of the 7th Workers Party Congress in Pyongyang Getty Images

South Korea's president said on Monday there were signs of "serious cracks" in the North's ruling elite class, and that Pyongyang could carry out some action to divert public attention away from such domestic problems.

Analysts said the well-publicised defection of Thae Yong Ho was an embarrassment to the North Korean government of leader Kim Jong-un, but would not weaken the unity of the country's elite class.

Previous South Korea-US military drills have brought threats of war from Pyongyang.

North Korea has already boosted such war rhetoric because of the planned deployment of the US Terminal High-Altitude Area Defence system in South Korea, which Washington and Seoul says is needed because of the increasing North Korean threats.