Ukrainian citizens endured the first day of nationwide scheduled power outages since the war began eight months ago so repairs could be made to damaged or destroyed energy plants as winter approaches.
"We can confirm that Russian military personnel based in Crimea have been piloting Iranian UAVs and using them to conduct kinetic strikes across Ukraine, including in strikes against Kyiv in recent days," US State Department spokesman Ned Price told a daily briefing with reporters, referring to unmanned aerial vehicles, or drones.
"We assess that ... Iranian military personnel were on the ground in Crimea and assisted Russia in these operations," Price said. He said "we do have credible information" but he did not provide evidence.
Russia seized Crimea from Ukraine in 2014 and has used the southern peninsula to train soldiers and reopen Soviet-era military bases as part of the invasion of its neighbour.
There was no immediate public reaction from Tehran to the US accusations but Iran has denied the drones are Iranian-made. Russia has also denied using Iranian drones in Ukraine.
Russia's defence and foreign ministries did not immediately respond to requests for comment.
White House national security spokesperson John Kirby told reporters that Washington was going to pursue all means to "expose, deter and confront" Iran's supply of munitions to Russia, including more sanctions, while also considering air defences for Ukraine.
European Union members have agreed on new measures against Iran, the bloc said, while Britain imposed sanctions on senior military figures and a firm it said were involved in the supply of Iranian drones to Moscow.
"Iran and Russia, they can lie to the world, but they certainly can't hide the facts, and the fact is this: Tehran is now directly engaged on the ground," Kirby said, without providing details.
Ukrainian Foreign Minister Dmytro Kuleba said on Twitter he had held detailed discussions with Israeli Prime Minister Yair Lapid on a request for air and missile defence systems and technology. Lapid's office said the Israeli leader expressed "deep concern" about the military connection between Iran and Russia.
People across Ukraine were urged to use less power as the government enforced electricity curbs between 7am and 11pm on Thursday.
The first such restrictions included blackouts in some areas, and followed a barrage of Russian attacks that President Volodymyr Zelenskiy said had struck a third of all power plants.
The northeast region of Sumy went without water and some grocery shops in the capital Kyiv reported brisk sales of bottled water.
Russia's defence ministry said it was again targeting Ukrainian energy infrastructure, a strategy it has stepped up since the appointment this month of Sergei Surovikin - nicknamed "General Armageddon" by the Russian media - as commander of what Russia calls its "special military operation" in Ukraine.
In video addresses on Thursday, Zelenskiy, without providing evidence, accused Russia of preparing to cause a large-scale disaster in southern Ukraine.
Ukraine has information that Russian forces have mined the dam and units of the Kakhovska hydroelectric power station, Zelenskiy asserted, adding that 80 settlements, including the city of Kherson, could experience rapid flooding.
The Ukrainian military reported fierce fighting in the Beryslav district as their forces press their advance towards Kherson city, the only regional capital Russian forces have captured. Control of Kherson gives Russia a land route to Crimea and the mouth of the Dnipro river, which bisects Ukraine.
Four people were killed when Ukrainian rocket artillery struck a ferry crossing in Kherson city late on Thursday, the Moscow-appointed deputy regional governor said.
