Russia has used ballistic missiles and launchers supplied by North Korea in its war on Ukraine, the US has said.

National Security Council spokesman John Kirby called it a "significant and concerning escalation" relating to Pyongyang's support for Russia.

He said the US would raise the matter at the UN Security Council and impose additional sanctions on those working to facilitate arms transfers.

Moscow has denied any such collaboration.

Hours after the White House made the accusations, North Korean leader Kim Jong Un called for missile launch vehicle production to be expanded in the country.

The North Korean leader visited Russia to discuss potential military co-operation in September.

The US has previously accused Pyongyang of supplying Russia with weapons, but this is the first time US intelligence has shared details about ballistic missiles - self-guided rockets that can reach targets 900km (500 miles) away.

Speaking during a White House press briefing on Thursday, Mr Kirby said Russia's procurement of ballistic missiles from North Korea was a direct violation of numerous UN Security Council resolutions.