Kyiv has been pleading with its allies to lift restrictions and allow it to strike what it calls "legitimate" military targets deep in Russian territory, like the air bases used by Russian planes in their relentless bombardment of Ukraine.

The comments by Ambassador Vassily Nebenzia came hours before President Joe Biden was to meet with visiting British Prime Minister Keir Starmer.

Some reports say the US leader is poised to endorse Kyiv's use of long-range British missiles against Russian military targets.

"If the decision to lift restrictions is really taken," Nebenzia warned fellow members of the UN Security Council, "that will mean that from that moment, NATO countries are conducting direct war with Russia.

"In this case, we will have to, as you understand, take the relevant decisions, with all the consequences of all this that the Western aggressors would incur."

He thus underscored a warning Thursday from President Vladimir Putin, who told state television that the use of long-range missiles against Russia would mean NATO was "at war" with Moscow.

Use of the missiles, Nebenzia said, would mean that "NATO will be a direct party to hostilities against a nuclear power; I think you shouldn't forget about this and think about the consequences."

US officials responded by again accusing Russia of receiving ballistic missiles from Iran.

Russia this month received hundreds of such missiles from Iran, said US ambassador Robert Wood, adding that the transfer "represents a dramatic escalation and a highly destabilizing development, one that is likely to increase the suffering for the Ukrainian people."

He accused North Korea of delivering missiles and munitions to Russia, and said China had provided Moscow with weapons components.

"It is clear there is an urgent need to help Ukraine defend itself from missile strikes, attacks which undermine the principles of the UN Charter and threaten international peace and security," Wood said.