In new evidence of the damage inflicted in the Ukrainian counter-offensive, video posted on social media on Friday and verified by Reuters showed a convoy of about 15 burnt-out Russian military trucks spaced out along a highway in the Kursk region.
Some contained bodies.
Soldiers push a damaged cars after a Russian missile hit a supermarket in Kostiantynivka, Donetsk. (AP PHOTO)
The acting governor of Kursk region, Alexei Smirnov, said drone debris had fallen on a power substation near Kurchatov, site of one of Russia's largest nuclear power stations with four reactors. Power to the area was cut for a time.
The head of the United Nations' International Atomic Energy Agency urged both sides to show restraint in view of the proximity of the conflict to the station, 60km from the border.
Russian diplomats in Vienna told the IAEA that fragments, possibly from downed missiles, had been found, though there was no evidence of an attack on the station.
Ukrainian forces broke across the border on Tuesday in a thrust that caught the Russian military by surprise after months of gradual advances in eastern Ukraine by Moscow's forces.
Politicians and the military are referring to a Ukrainian "invasion", nearly two and a half years after Russia launched its own full-scale invasion of its neighbour.
Authorities declared a federal state of emergency in Kursk.
Two days after Military Chief of Staff Valery Gerasimov reported to President Vladimir Putin that the advance had been halted, Russia's defence ministry said its forces "continue to repel an attempted invasion by the Armed Forces of Ukraine into the territory of the Russian Federation".
Interfax news agency quoted the ministry as saying that Russia was sending in columns of reinforcements with Grad multiple-launch rocket systems, artillery and tanks.
The video purporting to show Ukrainian forces in control of a gas measuring facility run by Russian concern Gazprom in the town of Sudzha was Ukraine's first pictorial acknowledgement of its troops' advance into Kursk region.
"The town is controlled by the Armed Forces of Ukraine, the town is calm, all buildings are intact," a soldier in the video said, adding that the "strategic Gazprom facility" was under the control of a Ukrainian battalion.
Reuters could not verify the video and the Ukrainian military's General Staff made no comment.
President Volodymyr Zelenskiy has maintained a strict silence on the operation, though he dropped some clear hints on Thursday, without referring to Kursk.
He praised his army's ability "to surprise". And in his nightly video address, he thanked army units who had taken Russian servicemen prisoner, to be used in later negotiations.
"This is extremely important and has been particularly effective over the past three days," he said.
The United States announced a new $US125 million ($A190 million) package of aid for Ukraine, including Stinger missiles, artillery ammunition, and anti-armour systems. Zelenskiy expressed thanks, saying the equipment was "vital for our forces to counter Russian assaults".
Soldiers clear rubble after a Russian missile hit a supermarket in Kostiantynivka, Donetsk. (AP PHOTO)
Russia's defence ministry released its own video which it said showed a drone destroying a Ukrainian tank and howitzer near Sudzha. Reuters was able to verify the location.
The ministry said that in the previous 24 hours, Russian troops, air strikes and artillery had "suppressed raid attempts by enemy units deep into Russian territory in the Kursk direction".
It said that Ukraine had lost up to 945 soldiers and 102 armoured vehicles in total during the Kursk fighting, without mentioning any losses on the Russian side.
