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 Zelensky praised his army on Thursday for its ability "to surprise" and achieve results. He did not explicitly reference Kursk.

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.

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.