Fighting has broken out on the streets of Kyiv as Russian and Ukrainian forces battle for control of the capital city.
Ukraine's Interior Ministry said "active fighting" is taking place in the city as it warned residents to be careful.
Images are also coming out of Kyiv showing a residential building which has been hit by a missile or rocket.
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The images, some of which have been published on Facebook by Ukraine's Internal Affairs Ministry, show residents being evacuated from the building, which has had a large hole blown in it on one corner.
Ukrainian President Volodymyr Zelenskyy remains in the capital, despite being urged by the United States to take up an offer to evacuate the city.
Mr Zelenskyy said "The fight is here; I need ammunition, not a ride," according to a senior American intelligence official with direct knowledge of the conversation, who described him as upbeat.
The President later posted a video on Twitter of him outside his office in Kyiv, vowing Ukraine will not lay down weapons and will instead continue to fight off the Russian advances.
"We will not lay down any weapons," he said in the early-morning video.
"We will defend our state because our weapons are our truth, and our truth is that this is our land, our country, our children, and we will defend it all."
In a later post, Mr Zelenskyy said he had talked with French President Emmanuel Macron, and that weapons are "on their way to Ukraine".
Battle for Kyiv begins
The fight to hold back Russia's assault on the capital intensified in the early hours of Saturday morning (local time, Saturday afternoon Australian time).
"This night will be very difficult, and the enemy will use all available forces to break the resistance of Ukrainians," Mr Zelensky warned in a late-night video message.
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"This night we have to stand ground. The fate of Ukraine is being decided right now."
Videos from eyewitnesses showed explosions taking place in an area north-west of Kyiv, an area where there is a military base.
CNN teams in the capital also reported hearing loud explosions to the west and south of the city. Shortly afterwards, Ukraine's State Service of Special Communications said clashes were underway in an eastern suburb, as Russian forces closed in on the capital from multiple sides.
Meanwhile, the Ukrainian armed forces reported heavy fighting around the city of Vasylkiv, some 30 kilometres southwest of Kyiv.
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"Heavy fighting is currently underway in the town of Vasylkiv in the Kyiv region, where the occupiers are trying to land a landing party," the armed forces said.
Russia's military claimed its forces had already staged a "successful landing operation" to capture the strategically important Hostomel airfield on Kyiv's outskirts.
As Russian troops advance, US intelligence officials are concerned Kyiv could fall under Russian control within days, according to two sources familiar with the latest intelligence.
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However, the latest British defence intelligence assessment said Russia had made "limited progress" in its attack.
"Ukraine retains control of key cities. Ukrainian MOD reports that Russian forces have arrived in the suburbs of Kyiv," the UK Ministry of Defence tweeted.
For now, Ukraine's democratically elected government remains intact but President Zelensky warned that "enemy sabotage groups" had entered this city and he is their number one target.
"They want to destroy Ukraine politically by destroying the head of state," he said.