Civilians will need to be dug from bunkers under a steel works that is the last holdout of resistance in Ukraine's Mariupol, President Volodymyr Zelenskiy says, after a Russian bombardment that has littered the area with concrete debris.
Having failed to capture the capital Kyiv in the early weeks of an invasion that has killed thousands and flattened towns and villages, Russia has accelerated attacks in southern and eastern Ukraine, including on the Azovstal steel works in Mariupol.
Ukraine's resistance at the plant underscores Russia's failure to capture major cities in a war that has united Western powers in arming Kyiv and punishing Moscow with sanctions. Sweden and Finland are expected to decide very soon whether to join NATO, in what would be a major historical shift.
Russia's military promised to pause its activity in Azovstal during Thursday daytime and the following two days to allow civilians to leave, after what Ukrainian fighters described as "bloody battles" prevented evacuations on Wednesday. The Kremlin said humanitarian corridors from the plant were in place.
Ukrainian officials believe about 200 civilians remain trapped along with fighters in the network of underground bunkers at the sprawling Soviet-era Azovstal complex.
In an early morning address on Thursday, Zelenskiy said Ukraine stood ready to ensure a ceasefire in Mariupol, a port city that after a weeks-long siege Russia controls - apart from the steel works.
"It will take time simply to lift people out of those basements, out of those underground shelters. In the present conditions, we cannot use heavy equipment to clear the rubble away. It all has to be done by hand," he said.
Ukraine's military general staff said the assault on the plant included air support, and pictures released by Russian-backed fighters appeared to show smoke and flames enveloping it.
Mariupol has been an important target in Russia's efforts to cut Ukraine off from its Black Sea grain and metals export routes, as well as to link Russian-controlled territory in the east of the country to Crimea, seized by Moscow in 2014.
The United Nations and Red Cross evacuated hundreds of people from the city and other areas this week.
Nobody from Azovstal was among more than 300 civilians evacuated on Wednesday from Mariupol and other areas in southern Ukraine, the UN humanitarian office said.
Russian President Vladimir embarked on the war partly to counter the expansion of the NATO alliance, which Ukraine also wants to join.
Sweeping sanctions from the United States and its European allies have hobbled Russia's $US1.8 trillion ($A2.5 trillion) economy while billions of dollars worth of military aid has helped Ukraine frustrate the invasion.
The United States has provided details on the location of Russia's mobile military headquarters, allowing Ukrainian forces to strike those targets and kill Russian generals, the New York Times reported on Wednesday, citing senior US officials.
Ukrainian officials said they have killed about 12 Russian generals on the battlefield, the newspaper said. The Pentagon and the White House did not immediately respond to Reuters' requests for comment on the report.
A source said European Union envoys could reach a deal as early as Thursday on a plan to phase out imports of Russian crude oil within six months and refined products by the end of this year. It requires agreement by all 27 EU governments.
The plan also targets Russia's top bank, its broadcasters, and hundreds of individuals. It would follow US and British oil bans and be a watershed for the EU, the world's largest trading bloc, which remains dependent on Russian energy.
The Kremlin said Russia was weighing responses to the EU plan, which it said would be costly for European citizens.
Ukraine and Russia said fighting had been heavy across the south and east over the past day.
More than five million Ukrainians have fled abroad since Putin launched the invasion on February 24. Millions more are displaced inside Ukraine.
Russia calls its actions a "special operation" to disarm Ukraine and protect it from fascists. Ukraine and the West say the fascist allegation is baseless and that the war is an unprovoked act of aggression.