Ukrainian authorities say that Russian troops have entered Ukraine’s second-largest city of Kharkiv and fighting is underway in the streets.

Oleh Sinehubov, the head of the Kharkiv regional administration, said Sunday that Ukrainian forces were fighting Russian troops in the city and asked civilians not to leave their homes.

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Russian troops approached Kharkiv, which is located about 20 kilometres (12.4 miles) south of the border with Russia, shortly after Moscow launched its invasion of Ukraine on Thursday. But until Sunday, they remained on its outskirts without trying to enter the city while other forces rolled past, pressing their offensive deeper into Ukraine.

Videos posted on Ukrainian media and social networks showed Russian vehicles moving across Kharkiv and a light vehicle burning on the street.

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Russia unleashed a wave of attacks on Ukraine targeting airfields and fuel facilities in what appeared to be the next phase of an invasion that has been slowed by fierce resistance. The US and EU responded with weapons and ammunition for the outnumbered Ukrainians and powerful sanctions intended to further isolate Moscow.

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Huge explosions lit up the sky early on Sunday south of the capital, Kyiv, where people hunkered down in homes, underground garages and subway stations in anticipation of a full-scale assault by Russian forces.

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Flames billowed into the air before dawn from an oil depot near the Zhuliany airport, about 25 miles (40 kilometres) south of the capital, according to President Volodymyr Zelensky’s office and the mayor of the nearby town of Vasylkiv.