Tehran and Moscow have denied Western accusations that Russia is using Iranian drones to attack targets in Ukraine, where officials warned on Friday of a winter-long power deficit after repeated Russian attacks on its energy infrastructure.
Two senior Iranian officials and two Iranian diplomats told Reuters in October that Iran had promised to provide Russia with surface-to-surface missiles as well as more drones.
White House national security spokesman John Kirby told reporters Washington was very concerned about the "deepening and burgeoning defence partnership" between Iran and Russia, and would work to disrupt that relationship, including on drones.
Washington was sending a $US275 million ($A405 million) package of aid to Ukraine to strengthen air defences and defeat drones, he said.
Britain's UN ambassador Barbara Woodward said Iran had sent hundreds of drones that Russia had used in Ukraine.
"Russia is now attempting to obtain more weapons, including hundreds of ballistic missiles," she told reporters. "In return, Russia is offering Iran an unprecedented level of military and technical support."
The Iranian and Russian missions to the United Nations did not immediately respond to requests for comment.
Russian President Vladimir Putin earlier said Moscow would probably do a deal over Ukraine one day but that Russia's near-total loss of trust in the West would make an eventual settlement, which he did not elaborate on, much harder to reach.
Russia has clamped down on dissent since it invaded Ukraine in February, and a Moscow court on Friday sentenced opposition politician Ilya Yashin to eight-and-a-half years in prison on charges of spreading "false information" about the army.
Yashin had discussed in a YouTube video evidence uncovered by Western journalists of Russian war crimes in Ukraine. Moscow denies committing war crimes. In a post on his Telegram channel, Yashin urged supporters to continue opposing the war.
On the ground in Ukraine, the entire front line in the east of the country was being shelled, said the governor of the Donetsk region, which is partly occupied by Russia. Five civilians were killed and two wounded in Ukrainian-controlled areas, the governor said.
President Volodymyr Zelenskiy said Ukrainian forces were standing their ground in the Donbas, made up of Donetsk and Luhansk regions, despite huge difficulties, but Russian forces had reduced the town of Bakhmut to ruins.
"The situation on the front line remains very tough," he said in an evening address. "I thank all our heroes, all soldiers and commanders who are... repelling assaults and inflicting significant losses on the enemy."
Reuters was not able to verify those battlefield reports.
Putin earlier repeated an accusation that the West was "exploiting" Ukraine and using its people as "cannon fodder" in a conflict with Russia, and said the West's desire to maintain its global dominance was increasing risks.
"They deliberately multiply chaos and aggravate the international situation," Putin said in a video message to a summit of defence ministers from the Shanghai Co-operation Organisation and a group of ex-Soviet states.
He later criticised France and Germany, which in 2014 and 2015 brokered ceasefire accords between Kyiv and Russian-backed separatists in eastern Ukraine, saying they had betrayed Moscow by supplying Ukraine with weapons.
"The question of trust arises. And trust of course is almost at zero... But nevertheless, in the final analysis we have to come to agreements. I have already said many times that we are ready for these agreements," Putin said.
US Defense Secretary Lloyd Austin criticised Putin on Friday as he voiced concern about Russia's expanding nuclear arsenal. His comments came hours after Putin vowed that any country that dared attack Russia with nuclear weapons would be wiped from the face of the earth.
"As the Kremlin continues its cruel and unprovoked war of choice against Ukraine, the whole world has seen Putin engage in deeply irresponsible nuclear sabre-rattling," Austin said, speaking at US Strategic Command, which oversees America's nuclear forces.
