Ukraine's military chief said on Sunday its forces were doing all they can to hold the line in Kharkiv - with Moscow claiming at least five villages on Saturday and four on Friday - but they are facing a "difficult situation".

Russia's surprise assault on the Kharkiv region began on Friday, threatening to open up a new front and stretch Ukraine's already-outgunned and outmanned forces.

"Units of the defence forces are fighting fierce defensive battles, the attempts of the Russian invaders to break through our defences have been stopped," Oleksandr Syrskyi said.

"The situation is difficult, but the defence forces of Ukraine are doing everything to hold defensive lines and positions, (and) inflict damage on the enemy."

Ukrainian military spokesman Nazar Voloshyn said Russia's attack is focused on the towns of Vovchansk, near the Russian border, and Lyptsi, which is 20km north of the city of Kharkiv.

In a post-midnight report, the Ukrainian military said Russian forces had achieved "tactical success" with 14 of 22 attempted advances in the area.

But it said Russian troops were "taking no account of their own losses", with at least 100 reported dead.

Mr Voloshyn urged residents to stay calm, saying Russia is waging an information campaign to spread panic alongside its military assault.

"The population should remain calm... the defence forces are holding, the situation is under control," he added.

Sky News defence and security editor Deborah Haynes has been reporting from Vovchansk, where her team followed rescuers as authorities urged residents to evacuate.

Nearly 6,000 civilians have fled the region, according to Governor Oleh Syniehubov, while Kyiv sends reinforcements to try to plug a supply gap widened by delays in Western military aid.

Ukraine says several months of delays by the US Congress to vote through a huge aid package has cost its forces on the battlefield.

That help is now on its way, but UK foreign secretary Lord David Cameron told Sky News on Sunday it "would have been better if it came earlier".

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Putin seizes chance to hit Kharkiv

Describing the Kharkiv offensive as an "extremely dangerous" moment, he said "we must do everything we can to help the Ukrainians".

In 2022, Russian forces reached the city of Kharkiv's suburbs before being driven back to the border.

Over the border, at least 15 people have been killed in the Russian region of Belgorod, according to the emergencies ministry, after a block of flats was struck by a Soviet-era missile launched by Ukraine.

The building partly collapsed, with at least 10 storeys affected.

Later, as emergency services scoured the rubble for survivors, the roof collapsed and people ran for their lives with rubble falling behind them.

The blast, one of the deadliest attacks on the region, also injured 20 people.

Kremlin spokesperson Dmitry Peskov described the attack as "barbaric", while Russian foreign ministry spokeswoman Maria Zakharova said it showed the criminality of Ukraine and its backers.