Russian forces have launched an armoured ground attack near Ukraine's second city of Kharkiv in the northeast of the country and made small inroads, opening a new front in a war that has long been waged in the east and south.
Ukraine sent reinforcements as fighting raged in the border areas of the region, the defence ministry said, adding that Russia had pounded the frontier town of Vovchansk with guided aerial bombs and artillery.
"Russia has begun a new wave of counteroffensive actions in this direction," President Volodymyr Zelenskiy told a news conference in Kyiv.
"Now there is a fierce battle in this direction."
Ukraine had warned of a Russian build-up in the area, potentially signaling preparations for an offensive or an effort to divert and pin down Ukraine's overstretched and outnumbered defenders. It was unclear if Moscow would develop the attack.
Zelenskiy has said Russia could be preparing a big offensive push this spring or summer. Kyiv's forces were prepared to meet Friday's assault, but Moscow could send more troops to the area, he told reporters.
"At approximately 5 am, there was an attempt by the enemy to break through our defensive line under the cover of armoured vehicles," the defence ministry said.
"As of now, these attacks have been repulsed; battles of varying intensity continue."
Kharkiv region's governor said the length of the border and the settlements in it were a "grey zone" and confirmed active fighting taking place.
A senior Ukrainian military source who declined to be named said Russian forces had pushed one kilometre inside the Ukrainian border near Vovchansk.
The source said Russian forces were aiming to push Ukrainian troops as far back as 10km inside Ukraine as part of an effort to create a buffer zone, but that Kyiv's troops were trying to hold them back.
There was no immediate comment from Russia.
At least two civilians were killed and five more were injured during heavy Russian shelling of border settlements, said Oleh Synehubov, governor of Kharkiv region.
"All the enemy can do is to attack in certain small groups, you can call them sabotage and reconnaissance groups or something else, and test the positions of our military," he said on television.
In Vovchansk, a border town with a pre-war population of 17,000 that has dwindled to a few thousand, authorities said they were helping civilians evacuate from the settlement and surrounding areas due to the heavy shelling.
Ukraine chased Russian troops out of most of the Kharkiv region in 2022, following Russia's full-scale invasion in February of that year. But after weathering a Ukrainian counteroffensive last year, Russian forces are back on the offensive and slowly advancing in the Donetsk region that lies further south.
Ukrainian concerns grew in March over the Kremlin's intentions in the Kharkiv region when Russian President Vladimir Putin called for the creation of a buffer zone inside Ukrainian territory. He said this was needed to protect Russia from shelling and border incursions.
Since then, Kharkiv, which is particularly vulnerable because of its proximity to Russia, has been hammered by air strikes that have damaged the region's power infrastructure.
More than two years after its invasion, Russia has the battlefield momentum and Ukraine faces shortages of manpower and stocks of artillery shells and air defences.
