North Korea warns US: it could launch a nuclear strike on any day

North Korea has reportedly claimed it can strike the US mainland with a nuclear weapon at any time in a chilling letter to Russian President Vladimir Putin .

Kim Jong-un's secretive state has said it is prepared to launch a devastating strike against the United States if it is provoked.

At the weekend Putin called for calm as tensions between Washington and Pyongyang escalate.

The ominous letter was handed over by a North Korean delegation to Russia last month, Sputnik reports.

It is claimed to have been passed to Valentina Matvienko, leader of Russia's upper house, who then passed it to the Russian President.

Yesterday US President Donald Trump posted a tweet claiming Kim had insulted him by calling him old, and said he would never call the North Korean despot 'short and fat'.

Trump made the comment after attending the Asia-Pacific Economic Cooperation (APEC) summit in Vietnam.

In a series of tweets he also said Chinese President Xi Jinping was "upping sanctions" on North Korea in response to its nuclear and missile programs and that Xi wants Pyongyang to "denuclearize."

One of Trump's tweets read: "Why would Kim Jong-un insult me by calling me 'old,' when I would NEVER call him 'short and fat?' Oh well, I try so hard to be his friend - and maybe someday that will happen!"

Trump has traded insults and threats with Kim in the past amid escalating tensions over Pyongyang's nuclear and missile programs as North Korea races toward its goal of developing a nuclear-tipped missile capable of reaching the United States - something Trump has vowed to prevent.

North Korea conducted its sixth and most powerful nuclear bomb test on September 3, prompting another round of U.N. sanctions.

In September Kim described Trump as a "mentally deranged U.S. dotard'' whom he would tame with fire.

His comments came after Trump threatened in his maiden United Nations address to "totally destroy" the country of 26 million people if the United States were threatened.

After North Korea's Foreign Minister Ri Yong Ho addressed the U.N. General Assembly in September Trump tweeted: "Just heard Foreign Minister of North Korea speak at U.N. If he echoes thoughts of Little Rocket Man, they won't be around much longer!"

The North has vowed to never give up its weapons programs, saying they are necessary to counter hostility from the United States and its allies.

The United States has said that all options, including military, are on the table, although its preference is for a diplomatic solution.