North Korean leader Kim Jong-un in his New Year's Day address said on Monday that his country has completed its nuclear arsenal and he has the ignition button ready "always on" his desk.

"We achieved the goal of completing our state nuclear force in 2017," Kim said in a televised message broadcast by the North Korean state network.

"A button is always on my desk," the North Korean leader said, adding that "this is reality, not a threat".

He called for an increase in production of nuclear warheads and ballistic missiles for operational deployment.

He stressed the need to "mass-produce nuclear warheads and ballistic missiles and accelerate their deployment," the South Korean news agency Yonhap quoted Kim as saying.

Kim also said that his country's nuclear forces have gained a powerful deterrent against the US and that Pyongyang's weapons were capable of hitting all of its mainland territory.

Kim also urged Washington and Seoul to end their joint military manoeuvres, which the regime criticised as an attempt to invade its country, and extended his hand to Seoul, saying that North and South Korea must improve their relations.

Throughout 2017, North Korea intensified its weapons tests with the launch of about 20 missiles, three of which were intercontinental, and completed its sixth and most powerful nuclear test to date in September.

The regime's repeated weapons tests have triggered a record number of sets of UN sanctions against the Asian country, four in one year.