Russia moved extra tanks, artillery and rocket systems to its southern Kursk region on Friday as it battled for the fourth straight day to end a shock incursion by Ukrainian forces.
In new evidence of the damage inflicted since the start of the Ukrainian counter-offensive, video posted on social media 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 containing dead bodies.
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.
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 Ukrainian military has remained silent about the offensive, though President Volodymyr Zelenskyy praised his army on Thursday for its ability "to surprise" and achieve results. He did not explicitly reference Kursk.
Ben Barry, land warfare analyst at the International Institute for Strategic Studies (IISS), said that while its wider strategic goals remained unclear, Ukraine had exposed Russian shortcomings and overturned the conventional wisdom on the war that the battlefield was "transparent" and neither side could advance without heavy losses.
"They clearly have achieved a degree of surprise which suggests that Russia's ability to do intelligence, surveillance and reconnaissance is inadequate," he said in a phone interview.
Destroyed trucks
A Ukrainian Telegram channel that posted the video of the destroyed Russian trucks said they had been hit by a US-supplied HIMARS rocket system.
Russian bloggers also blamed a HIMARS strike, and one said whoever had given the order for military vehicles to move in exposed columns was an "asshole" who should be shot.
Reuters was not able to establish how the vehicles were destroyed. MASH, a Russian news outlet with contacts in the security services, said the video was filmed and passed on to the Ukrainian channel by a local man who had subsequently been arrested on suspicion of spying.
Russia's defence ministry released its own video which it said showed a drone destroying a Ukrainian tank and howitzer near the town of Sudzha. Reuters was able to verify the location.
The ministry's statement said that over the past 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.
Reuters could not verify the battlefield accounts. On Wednesday Gerasimov had said the Ukrainian attack was mounted by up to 1,000 troops.
The Institute for the Study of War said in an overnight report that geolocated footage and Russian accounts indicated that Ukrainian forces had "continued rapid advances" further into Kursk region on Thursday.
There were some unconfirmed reports from Russian sources of Ukrainians pushing as deep as 35 km (22 miles) from the border at the furthest point.
Rybar, a Russian military blog, said Ukrainian units had been entering village after village and staging ambushes against arriving Russian reinforcements.
Kursk acting governor Alexei Smirnov issued a series of security alerts to residents, urging them to shelter from potential missile attacks. Authorities declared a federal state of emergency in the region.
The Russian rouble was down 2.5 per cent against the dollar and traders said the Ukrainian attack on Kursk region was one of the factors behind the currency's weakness.
London-based analyst Barry said there were at least half a dozen possible motives behind the Ukrainian attack, including to draw away Russian forces from the battlefront in eastern Ukraine or to seize and hold Russian territory to use as a bargaining chip.
He said Kyiv might also be seeking to lure Russian warplanes towards Kursk, where they might be vulnerable to its ground-based air defence missiles, and divert them from conducting operations against Ukraine's newly delivered F-16 fighter jets from the West.
The unexpected ground offensive on Russian soil, he said, was also partly about "being seen to bring the war home to Russia, and undermining Putin and the military's credibility".