Scores of Ukrainian civilians including women and children were trapped in underground bunkers at a steel works in the ruined port city of Mariupol on Thursday, although Russian president Vladimir Putin said Russia was ready to allow them to leave safely.
Mr Putin told Israeli prime minister Naftali Bennett in a phone call that Kyiv should order Ukrainian fighters defending the besieged Azovstal plant to disarm, and Russia was still prepared to provide safe passage out for civilians.
Ukrainian defenders at the site have clung on desperately for weeks, and while some civilians have made it to safety via humanitarian corridors, about 200 others, Ukrainian officials say, remain inside with little food, water or medicine.
The Kremlin earlier denied that Russian forces were storming the Soviet-era plant, referring to an April 21st order from Mr Putin that they should seal it off but not venture inside its labyrinth of underground tunnels.
But a Ukrainian fighter who said he was holed up in the vast complex - the last part of the city still in Ukrainian hands - accused Russian forces of breaching the plant's defences for a third day in violation of an earlier pledge by Moscow to pause military activity to permit civilian evacuations.
"Heavy, bloody fighting is going on," Captain Sviatoslav Palamar, a deputy commander of Ukraine's Azov Regiment, said in a video posted online.
"Yet again, the Russians have not kept the promise of a ceasefire and have not given an opportunity for the civilians who seek shelter ... in basements of the plant to evacuate," he said. Reuters could not independently verify his account or from where he was speaking.
Ukraine's defence of Azovstal has underlined Russia's failure to take major cities in a 10-week-old war that has united Western powers in arming Kyiv and punishing Moscow with sanctions.
In what would be a major historic shift sure to infuriate Moscow, Sweden and Finland may shortly decide to join Nato.
Humanitarian corridors
Russia's military promised to pause its activity in Azovstal during Thursday daytime and the next two days to allow civilians to leave, after fighting prevented evacuations from the plant on Wednesday. The Kremlin said humanitarian corridors from the plant were in place.
Pictures released by Russian-backed fighters appeared to show smoke and flames enveloping the complex.
In an early morning address, Ukrainian president Volodymyr Zelenskiy said Ukraine stood ready to ensure a ceasefire.
"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," Mr Zelenskiy said.
Ukraine's military general staff said the assault on the plant included air support.
The chief commander of Ukraine's armed forces said they needed multiple launch rocket systems to defend against resumed Russian cruise missile strikes around the country. Russia has in recent days attacked railways, weapons dumps and fuel depots.
The US Congress is debating an aid package for Ukraine worth $33 billion, largely for weapons. If it gets more supplies, Ukraine could launch a counter-offensive in mid-June, an adviser to Mr Zelenskiy said.
Mariupol is an important target in Russian efforts to cut Ukraine off from its coastal 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.
Nato enlargement
Sweden and Finland, which shares a 1,300km border with Russia, stayed out of Nato during the Cold War, but Moscow's invasion of Ukraine has prompted them to rethink their security needs.
Mr Putin embarked on the war partly to counter the expansion of the Nato alliance.
Sweeping sanctions from Washington and European allies have hobbled Russia's $1.8 trillion economy, while billions of dollars worth of military aid has helped Ukraine frustrate the invasion.
European Union countries are "almost there" in agreeing the bloc's proposed new package of sanctions against Russia, including an oil embargo, the bloc's foreign policy chief Josep Borrell said.
The Kremlin said Russia was weighing responses to the plan.
Battles
Ukraine and Russia said fighting had been heavy across the south and east over the past day.
Ukrainian authorities reported shelling of towns near a frontline that divides territory it holds in the Donetsk and Luhansk regions from land held by Russian-backed separatists.
The Ukrainian air force said it had downed three Russian cruise missiles and four aircraft, including two Sukhoi fighter jets, while Russia said it had killed 600 Ukrainian soldiers overnight. Reuters could not independently either report.
Ukrainian officials have warned that Russia might step up its offensive before May 9, when Moscow commemorates the Soviet Union's victory over Nazi Germany in World War Two.
Russia calls its actions a "special military 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.
More than 5 million Ukrainians have fled abroad since the start of the invasion.
