Ukrainian forces are battling Russian troops on the streets of Ukraine's second city Kharkiv, according to local officials.
Regional governor Oleh Sinegubov said light vehicles had entered the city, as he urged its 1.4 million residents to stay indoors.
The advance came after missiles rained down overnight.
A nine-storey residential tower was hit, emergency services said.
The building was severely damaged and an elderly woman was killed, but rescuers said about 60 people were spared injury as they had taken refuge in the basement.
Russian troops also blew up a natural gas pipeline nearby, according to a Ukrainian state communications agency.
The fighting across Ukraine has resulted in at least 240 civilian casualties, including 64 deaths, according to the Office of the United Nations High Commissioner for Human Rights (OCHCR).
It added that damage to homes and critical infrastructure has left hundreds of people without access to water and electricity.
The BBC has verified a video on the Telegram messaging platform which shows Russian trucks driving into Kharkiv. It comes after an oil terminal outside the capital Kyiv was attacked by missile, prompting toxic air warnings. The massive explosion in Vasylkiv, 18 miles (30km) south of Kyiv, saw flames light up the night sky. Those nearby were warned to close their windows - though many in Kyiv are already sheltering underground. A strict curfew in the capital is in place until Monday. BBC producer Kathy Long said Sunday dawned cold and quiet in Kyiv, with police, the military and armed volunteers the only people visible on the streets. But the blue and yellow national flag still flies on many of the ornate buildings that make up Kyiv's skyline, as the country enters its fourth day under Russian operation. In underground car parks, metro stations and basements, thousands are waiting to assess the damage the darkness brought.
Meanwhile in the northeastern city of Okhtyrka, the local governor said at least six Ukrainians - including a seven-year-old girl - had died as a result of a Russian attack on Friday.
A kindergarten and an orphanage were among the buildings reportedly hit, which Russia has denied.
Ukraine's Foreign Minister Dmytro Kuleba has alleged war crimes and called for an investigation by the International Criminal Court. In Koryukivka, in Chernihiv region, residents were seen stopping advancing Russian tanks by walking them back en masse.
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Tens of thousands of Ukrainians have been pouring towards neighbouring countries to escape the war. More than 115,000 have crossed into Poland alone - some travelling for more than two days, and others joining queues 15km (10 miles) long at border points. Those fleeing are mostly women and children, as all Ukrainian men aged 18 to 60 are being told to stay and fight. Despite the Russian onslaught, Ukraine's defences have held for another night.
According to a US-based war monitor, Russia has "failed to encircle and isolate Kyiv with mechanised and airborne attacks as it had clearly planned to do".
The Study of War's latest assessment said Russian attacks on other cities in the northeast and east have failed because they were "poorly designed and executed", and they faced "more determined and effective resistance than expected" from Ukrainian forces.
The monitor added that Russia's successes in the south of Ukraine posed the most danger.
