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.

"We will demand that Russia be held accountable for yet again violating its international obligations," he said.

He also said the US believed Russia was planning on purchasing close-range missiles from Iran, but that it had not done so yet.

The UK said it "strongly condemns" Russia's use of ballistic missiles sourced from North Korea in Ukraine.

"North Korea is subject to a robust sanctions regime, and we will continue to work with our partners to ensure that North Korea pays a high price for supporting Russia's illegal war in Ukraine," a spokesperson for the Foreign, Commonwealth and Development Office said.

In his briefing, Mr Kirby also urged US Congress to approve extra US funding for Ukraine "without delay".

"The most effective response to Russia's horrific violence against the Ukrainian people is to continue to provide Ukraine with vital air defence capabilities and other types of military equipment," he said.

"Iran and the DPRK [North Korea] are standing with Russia. Ukrainians deserve to know that the American people and this government will continue to stand with them."

The last US military aid package to Ukraine, worth some $250m (£195m), was approved by the White House on 27 December.

Talks on further funding have stalled in Congress because of a lack of support among Republicans, who insist that tougher security measures on the US-Mexico border must be part of any military aid deal.

Ukraine has warned that its war effort and the country's public finances are at risk if further Western aid does not come soon.