Russia launched a surprise ground offensive into Ukraine's northeast Kharkiv region on Friday, putting its forces on the attack in a border zone Moscow's troops were pushed back from nearly two years ago.
Civilians were told to flee border areas as heavy fighting was raging between Russian and Ukrainian forces, which are struggling with ammunition and manpower shortages.
"Russia launched a new wave of counter-offensive actions in this area. Ukraine met them there with our troops, brigades and artillery... Now there is a fierce battle under way in this area," President Volodymyr Zelensky told a briefing.
A high-ranking Ukrainian military source said Russia had advanced into Ukraine by one kilometre (0.6 miles), and was trying to "create a buffer zone" to prevent attacks into Russian territory.
If Russia's advances are confirmed, it would represent the Kremlin's biggest land operation in the region since it sent thousands of troops across the border in February 2022.
Ukraine reported repelling Russia's incursion but that "fighting of varying intensity" was ongoing and that Russia had launched aerial strikes into an area near the border.
The attack was launched around 5:00 am (0200 GMT), with Russia trying to break through Ukraine's lines under the cover of armoured vehicles, the Ukrainian defence ministry said.
The Russian defence ministry has not provided any comment but Russian military bloggers reported active fighting in the area.
'Massive shelling'
A local official said there was "massive shelling" in Vovchansk, a town of about 3,000 some five kilometres (three miles) from the Russian border, and that evacuations were underway there and in nearby areas.
At least two people were killed in Russian shelling of the region on Friday, local governor Oleg Synegubov said. "Enemy shelling from various types of weapons has been intensifying in Kharkiv region in the northern area for the last day," he wrote on Telegram.
"In Kharkiv region, evacuation routes have been developed since 2022, and a system of humanitarian aid distribution, temporary resettlement," he said.
Vitaliy Ganchev, a Russian-installed official for the Kharkiv region, confirmed there was fighting near the border and urged civilians to take shelter. "There is fighting on several parts of the line of contact, including in the border areas of Kharkiv region," he said on Telegram.
"In this regard, I ask residents living in these areas to be careful and not to leave shelters without an urgent need," he added.
Russia claimed to have captured at least two villages in the region this week, pressing its advantage against outgunned and outmanned Ukrainian troops.
President Vladimir Putin warned in March he was considering creating a "sanitary zone" in Ukrainian territory adjoining Belgorod, a Russian region frequently targeted by Ukrainian shelling.
Ukrainian shelling on Russian border villages killed at least two people on Thursday, the governors of Russia's Belgorod and Kursk border regions said.
Pro-Ukrainian militias also claimed earlier this year to have stormed Russia's frontier in a series of brazen raids, a source of embarrassment for the Kremlin.