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Russian troops entered Ukraine's second city of Kharkiv and fighting was under way on Sunday, the head of the regional administration said on the fourth day of Moscow's military operation.
"The Russian enemy's light vehicles broke into the city of Kharkiv," said Oleg Sinegubov, chairman of the Kharkiv Regional State Administration, in a Facebook post. "The Ukrainian armed forces are eliminating the enemy. We ask civilians not to go out."
Videos published by Anton Herashchenko, adviser to the interior minister, and Ukraine's State Service of Special Communications and Information Protection showed several light military vehicles moving along a street and, separately, a burning tank.
Russia unleashed a wave of attacks on Ukraine, aiming at airfields and fuel terminals, in what appeared to be the next phase of its military operations that have been slowed by fierce resistance.
The US and EU sent more weapons for the outnumbered Ukrainians and imposed sweeping sanctions intended to further isolate Moscow.
Large explosions lit up the sky early on Sunday south of the Ukrainian capital Kiev, where people sought shelter in homes, underground garages and metro stations in anticipation of a full-scale assault by Russian troops.
Flames rose into the sky before dawn from an oil depot near an airbase in Vasylkiv, where there had been intense fighting, said the town’s mayor.
Ukrainian President Volodymyr Zelenskyy’s office said another explosion occurred at the civilian Zhuliany airport.
Moscow ordered its troops to advance in Ukraine "from all directions", while the West responded late on Saturday with sweeping sanctions aimed at Russia's banking sector.
Ukrainian officials reported that 198 civilians, including three children, had been killed since Russia launched its military operation on Thursday, and said Russian saboteurs were active in Kiev, where explosions forced residents to take shelter underground.
Moscow said it fired cruise missiles at military targets, continuing the offensive after accusing Ukraine of having "rejected" talks.
Mr Zelenskyy’s office also said Russian forces blew up a gas pipeline in Kharkiv.
The government gave a warning that smoke from the enormous explosion could cause an “environmental catastrophe” and advised people to cover their windows with damp cloth or gauze.
Ukraine's army said it held back an assault on the capital, but was fighting Russian "sabotage groups" that had infiltrated the city.
"We will fight until we have liberated our country," Mr Zelenskyy said in a video message.
He earlier said Ukraine had "derailed" Moscow's plan to overthrow him and urged Russians to pressure President Vladimir Putin into stopping the conflict.
In retaliation for the invasion, the West said it would remove some Russian banks from the Swift bank messaging system, and froze central bank assets, dealing a blow to Russia's trade with most of the world.
Tens of thousands more are estimated to be displaced within Ukraine.
In neighbouring Romania, Olga, 36, was among hundreds to have crossed the Danube river with her three young children to safety.
"My husband came with us as far as the border, before returning to Kiev to fight," she said.
Thousands have made their way to Poland by train.
"Attacks were everywhere," said Diana, 37, who fled the Ukrainian capital. "My mother is still in Kiev."
As air raid sirens rang out in the capital, residents sought sanctuary in subway stations and cellars, while Mr Zelenskyy announced a baby girl had been born on the metro.
The city said anyone outside after 1500 GMT would be considered "members of the enemy's sabotage and reconnaissance groups".
The curfew will last until 8am on Monday.