Ukrainian President Volodymyr Zelensky has said there were "heavy battles" along the entire front line and vowed to quash a new major Russian ground assault.
Russia launched a surprise ground offensive into Ukraine's northeastern Kharkiv region this morning, making small advances into a border zone it was pushed back from nearly two years ago.
"There are heavy battles along the entire front line," Mr Zelensky said in a post on social media after hearing a report from his commander-in-chief this evening.
Ukraine was sending reinforcements to the Kharkiv region and responding to the Russian attack with artillery and drones, he said.
The Ukrainian leader also said Ukraine would meet the fresh Russian challenge with force, and called on Kyiv's Western backers to deliver more military aid.
"Along our state border and along our front line, we will invariably destroy the occupier in such a way as to thwart any Russian offensive intentions," he said in an evening video address.
"It is important that partners support our soldiers... with timely supplies," he added.
Civilians were told to flee and heavy fighting was reported as Ukraine's outgunned army rushed in reinforcements to defend the region, which has been mostly under Ukrainian control since September 2022.
"Russia launched a new wave of counter-offensive actions in this area. Ukraine met them there with our troops, brigades and artillery," Mr Zelensky told a briefing earlier.
"Now there is a fierce battle under way," he added.
A high-ranking Ukrainian military source said Russia had advanced into Ukraine by one kilometre and was trying to "create a buffer zone" in both the Kharkiv and neighbouring Sumy regions to prevent attacks into Russian territory.
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If Russia's advance is confirmed, it would represent the Kremlin's biggest land operation in that part of the battlefield since it sent thousands of troops across the border in February 2022.
Officials in Kyiv had for weeks warned Moscow might try and attack its northeastern border regions, pressing its advantage as Ukraine struggles with delays in Western aid and manpower shortages.
The US announced a new $400 million military aid package for Kyiv hours after the offensive began, as the White House seeks to supply military hardware as quick as possible.
Ukraine's military said it had halted Russia in its tracks but that "fighting of varying intensity" was ongoing.
"Reserve units have been deployed to strengthen the defence in this area of the front," it said.
Russia launched the attack around 2am Irish time, breaking through Ukraine's lines under the cover of armoured vehicles, according to Kyiv.
The Russian defence ministry has not provided any comment but military bloggers reported Moscow had made inroads.
"Active fighting is currently taking place in settlements one to two kilometres from the border with Russia," Kharkiv region governor Oleg Synegubov said.
"We urge people to evacuate in this area," he added.
A local official said there was "massive shelling" in Vovchansk, a town of about 3,000 people some 5km from the Russian border, and that evacuations were under way there and in nearby areas.
Russian shelling killed three people in the Kharkiv region and two others in the eastern region of Donetsk, another fighting hotspot, local prosecutors said.
"Enemy shelling from various types of weapons has been intensifying in Kharkiv region in the northern area for the last day," Mr Synegubov wrote on Telegram.
"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 as it seeks to gain ground before new Western aid arrives.
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.
