North Korea has threatened war with the United States after John McCain branded its leader Kim Jong-un a “crazy fat kid”.

The US Senator, speaking to MSNBC earlier this month, said: “China is the only one that can control Kim Jong-un, this crazy fat kid that’s running North Korea. They could stop North Korea’s economy in a week.”

The 74-year-old claimed Kim was irrational. “We’re not dealing even with someone like Joseph Stalin, who had a certain rationality to his barbarity,” he said.

North Korea’s state news agency KCNA released a statement from the Foreign Ministry, which said the remarks had “hurt the dignity” of the country.

Addressing both Mr McCain and Republican Senator Ted Cruz, who had urged North Korea to be designated a state-sponsor of terrorism, the statement read: “What they uttered to dare hurt the dignity of the supreme leadership of the DPRK is just a manifestation of their worst hostility toward the DPRK's ideology and social system and its people and a grave provocation little short of declaration of war against it.

"The service personnel and people of the DPRK are regarding the dignity of their supreme leadership as their life and soul.

"The US must know very well about how they react to any offensive acts against it. As such guys as John McCain and Ted Cruz made a provocation tantamount to declaration of war against the DPRK, the DPRK will take steps to counter it.

"They will have to bitterly experience the disastrous consequences to be entailed by their reckless tongue-lashing and then any regret for it will come too late.

Kim Jong-un: Successor battling a weight problem – he's too thin Show all 2 1 / 2 Kim Jong-un: Successor battling a weight problem – he's too thin Kim Jong-un: Successor battling a weight problem – he's too thin Pg-8-KimJong6-ap.jpg Kim Jong-il with his son Kim Jong-un at a military parade AP Kim Jong-un: Successor battling a weight problem – he's too thin Kim-Jong-un-afp.jpg Kim Jong-Un, youngest son of former North Korean leader Kim Jong-Il, sitting during a meeting with his father and visiting Lao President AFP

"They will have to be entirely responsible for their foolhardy tongue-lashing."

The statement ended with a warning that North Korea “will fulfil its sacred mission” of “dealing a merciless sledge-hammer blow at those daring hurt the dignity of the supreme leadership”.

Mr McCain ridiculed the response, tweeting: “What, did they want me to call him a crazy skinny kid?”

Kim’s weight appears to be a touchy subject in the secretive state. In November North Korean officials reportedly asked the Chinese government to ban social media users from calling its Supreme Leader “fatty”.