Moscow on Saturday mounted a "counter-terror operation" in three border regions adjoining Ukraine to halt Kyiv's biggest cross-border offensive in the two-and-a-half-year conflict.
Ukrainian units stormed across the border into Russia's western Kursk region on Tuesday morning in a shock attack and have advanced several kilometres, according to independent analysts.
Russia's army has rushed in extra troops and equipment, including convoys of tanks, rocket launchers and aviation units -- though neither side has given precise details on the extent of the forces they have committed.
At least 3,000 civilians have been evacuated from Russian border areas, where emergency aid and medical supplies have been ferried in, while extra trains to the capital Moscow have been put on for people looking to flee.
"The war has come to us," one woman who fled the border zone told AFP at a Moscow train station on Friday, declining to give her name.
Russia's army said Ukraine initially despatched around 1,000 troops, and more than two dozen armoured combat vehicles and tanks -- but it has since claimed to have destroyed around five times as many pieces of military hardware.
AFP could not verify those numbers and both sides have repeatedly been accused of inflating the number of enemy losses while downplaying their own setbacks.
Russia's national anti-terrorism committee said late Friday it was starting "counter-terror operations in the Belgorod, Bryansk and Kursk regions ... in order to ensure the safety of citizens and suppress the threat of terrorist acts being carried out by the enemy's sabotage groups."
Security forces and the military are given sweeping emergency powers during "counter-terror" operations.
Movement is restricted, vehicles can be seized, phone calls can be monitored, areas are declared no-go zones, checkpoints introduced, and security is beefed up at key infrastructure sites.
The anti-terrorism committee said Ukraine had mounted an "unprecedented attempt to destabilise the situation in a number of regions of our country."
It called Ukraine's incursion a "terrorist attack" and said Kyiv's troops had wounded civilians and destroyed residential buildings.
The health ministry said Friday that 55 civilians were in hospital, 12 in a serious condition.
Several Russian media outlets shared a video purporting to show residents from the Sudzha district of Kursk, where Ukraine's offensive has focused, appealing to President Vladimir Putin for help, warning that many were unable to evacuate.
Russia on Friday appeared to hit back at the incursion, launching a missile strike on a supermarket in the east Ukrainian town of Kostyantynivka that killed at least 14 people.
The US-based Institute for the Study of War said Saturday it believed Ukrainian forces had pierced around 13 kilometres (eight miles) into Russian territory, though cautioned assessing the position of Ukraine's troops was difficult.
Ukraine's leaders have remained tight-lipped on the operation.
The United States, Kyiv's closest ally, said it was not informed of the plans in advance.
But President Volodymyr Zelensky has appeared to tout his troops early successes, saying earlier this week that Russia must "feel" the consequences of the full-scale offensive it launched in February 2022.
On Friday he also thanked Ukrainian troops for the "replenishment of the exchange fund" -- language used to refer to the capture of Russian soldiers, who can later be swapped for captured Ukrainians.
"This is extremely important and has been particularly effective over the last three days," he said, again without making any specific reference to the Kursk incursion.
Russian military bloggers, who typically post more open, detailed and timely information than the defence ministry in Moscow, previously reported several Russian soldiers had been taken prisoner by Ukraine.
Russia's defence ministry published footage on Saturday of tank crews firing on Ukrainian positions in the Kursk region, as well as an overnight air strike, after it said Friday it had deployed yet more units to the border region.
Elsewhere on the frontline, Ukrainian officials said two were killed in the northeast Kharkiv region and one in the city of Kramatorsk.
The Ukrainian army on Saturday reported a reduced number of "combat engagements" inside Ukraine -- a possible sign that its incursion into Russia could be working to relieve pressure on other parts of the sprawling frontline where Moscow's troops had been advancing.
