UKRAINE has rushed reinforcements to the city of Kharkiv as an intense battle is underway after Putin launched a surprise new offensive.
Kyiv officials said they were expecting the tactical switch in the war by Moscow for weeks.
Russian forces pounded the town of Vovchansk with guided aerial bombs and artillery as Volodymyr Zelensky said a "fierce battle" is taking place.
The barrage killed at least one civilian and wounded five others, prompting authorities to begin evacuating about 3,000 people.
Ukraine rushed in reserve units as fighting raged in the border areas of the Kharkiv region, the defence ministry said.
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The ministry said: "At approximately 5am, there was an attempt by the enemy to break through our defensive line under the cover of armoured vehicles.
"As of now, these attacks have been repulsed; battles of varying intensity continue."
A senior Ukrainian military source said Russian forces had pushed 1km inside the Ukrainian border near Vovchansk.
The source said Russian forces were trying to push Ukrainian troops as far back as 10km inside Ukraine as part of a bid to create a "buffer zone" - but Kyiv's troops were trying to hold them back.
Russian military bloggers said the assault could mark the start of a Russian attempt to carve out a buffer zone that Putin vowed to create earlier this year to halt frequent Ukrainian attacks on Belgorod and other Russian border regions.
If Russia's advances are confirmed, it would be Vlad's biggest ground operation in the region since it sent thousands of troops across the border in February 2022.
According to Pravda, Putin's troops have captured four villages on the border - Streleche, Krasne, Pylne and Borysivka - and are now trying to advance in the direction of Vovchansk.
Vovchansk administration head Tamaz Hambarishvili told Ukraine's Hromadske Radio: "The entire town is under massive shelling now, it is not safe to stay here."
Vlad has reportedly amassed 40,000 troops and some 500 tanks along the northeastern border - close to the Kharkiv and Sumy regions.
The surprise invasion is thought to be tactical switch by Moscow that Ukrainian officials have been expecting for weeks as the war stretches into its third year.
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Zelensky has warned that Russia could be preparing a big offensive push this spring or summer.
He told a press conference in Kyiv that Ukrainian forces were prepared for the fight - but said Moscow could send more troops to the area.
"Russia launched a new wave of counter-offensive actions in this area," he said.
"Ukraine met them there with our troops, brigades and artillery.
"Now there is a fierce battle under way in this direction."
The Institute for the Study of War said it's unlikely Russia will be able to take Kharkiv - but opening a new front in the war could force Ukraine to send in more troops, leaving other areas vulnerable to attack.
In 2022, Ukraine chased Russian troops out of most of the Kharkiv region following Russia's full-scale invasion.
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.
'BUFFER ZONE'
In March, Vlad called for the creation of a buffer zone inside Ukrainian territory "to protect Russia" - sparking concerns over the Kremlin's intentions in the Kharkiv region.
Since then, Kharkiv has been battered by air strikes, damaging the region's power infrastructure.
The Russian defence ministry has not commented on Friday's attack - but Russian military bloggers reported active fighting in the area.
And Vitaliy Ganchev, a Russian-installed official in 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."
Russia has claimed to have captured at least two villages in the region this week.
Meanwhile, a Ukrainian long-range drone struck an oil refinery inside Russia on Friday, officials said - a day after what appeared to be the deepest strike by Kyiv's forces on Russian soil hit a petrochemical facility.
Ukraine has repeatedly targeted refineries, hoping to disrupt Vlad's war machine.
More than two years after its invasion, Russia has gained momentum on the battlefield as Ukraine faces shortages of manpower and stocks of artillery shells and air defences.
