Pyongyang has delivered more than 1,000 containers of "equipment and munitions" in recent weeks, National Security Council Spokesperson John Kirby said on Friday.

Washington released pictures that it said showed containers loaded with arms on a Russian-flagged ship before being moved by train to southwestern Russia.

The containers were shipped between 7 September and 1 October, between Najin, North Korea, and Dunay, Russia, according to the White House.

Speculation Pyongyang would start sending arms to Russia has been rife since North Korean leader, Kim Jong Un, visited Russia to meet President Vladimir Putin and see key military sites.

In return, North Korea wants sophisticated Russian weapons technologies to use in its nuclear weapons development programme, Mr Kirby said.

Satellite photos showing a sharp increase in rail traffic along the North Korea-Russia border were published last week by the Centre for Strategic and International Studies.

Images dated 5 October, the Washington thinktank said, showed "a dramatic and unprecedented level of freight railcar traffic" at the Tumangang Rail Facility, on the country's border with Russia.

More than 70 railcars were identified, the centre said, whereas only around 20 were visible over the previous five years.

The US has accused North Korea of previously providing ammunition, artillery shells and rockets to Russia.

Last month, after meeting Mr Putin, the North Korean leader demanded an exponential increase in his country's production of nuclear weapons.

It was time, he said, for Pyongyang to play a larger role in a coalition of nations confronting the US in a "new Cold War", North Korean state media reported.

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The Korean Central News Agency said Mr Kim made the comments during a two-day session of the country's parliament, which changed the constitution to include his policy of expanding the country's nuclear weapons programme.

Mr Kim deepened "comradely fellowship and friendly ties" with Mr Putin, during his six-day trip to Russia, North Korean state media said.

The two countries said they discussed boosting their defence ties but did not disclose any specific steps.