* Russian forces take town near Chernobyl at second attempt
* Russia signals military focus on eastern Ukraine
* Ukrainian president warns of Russia energy 'blackmail'
* US president meets Ukrainian ministers in Warsaw
Russian forces have taken control of a town where workers at the defunct Chernobyl nuclear plant live, the governor of region said on Saturday, and fighting was reported in the streets of the besieged southern port of Mariupol.
After more than four weeks of conflict, Russia has failed to seize any major Ukrainian city and Moscow signalled on Friday it was scaling back its military ambitions to focus on territory claimed by Russian-backed separatists in the east.
Intense fighting was reported in a number of places on Saturday, suggesting there would be no swift let-up in the conflict, which has killed thousands of people, sent nearly 3.8 million abroad and driven more than half of Ukraine's children from their homes, according to the United Nations.
Three explosions were heard near Lviv in western Ukraine, and a Reuters witnesses saw black smoke rising from the northeastern side of the city. The cause could not immediately be verified.
US President Joe Biden, visiting Nato ally Poland, called Russian President Vladimir Putin a "butcher". Biden said he was not sure Russia was changing its strategy in Ukraine to focus on efforts to "liberate" the breakaway eastern Donbass region, despite getting bogged down in some areas.
Russian troops seized Slavutych, which is close to the border with Belarus and is where workers at the nearby Chernobyl plant live, said Oleksandr Pavlyuk, the governor of Kyiv region.
He said Russian forces had fired into the air and thrown stun grenades to disperse residents who unfurled a large Ukrainian flag and shouted "Glory to Ukraine" in protest. Reuters could not independently verify the reports.
Slavutych sits just outside the so-called exclusion zone around Chernobyl, which was the site of the world's worst nuclear disaster in 1986. Ukrainian staff have continued to work at Chernobyl after the plant was seized by Russian forces soon after the start of the Feb. 24 invasion.
In the encircled southern city of Mariupol, Mayor Vadym Boichenko said the situation remained critical, with street fighting in the centre. Mariupol has been devastated by weeks of Russian fire.
In an address to Qatar's Doha Forum, Ukrainian President Volodymyr Zelenskiy compared the devastation in Mariupol to the destruction inflicted on the Syrian city of Aleppo by combined Syrian and Russian forces in Syria's civil war.
"They are destroying our ports," Zelenskiy said, warning of dire consequence if his country - one of the world's major grains producers - could not export its foodstuffs. "The absence of exports from Ukraine will deal a blow to countries worldwide."
Speaking via video link, he also called on energy producing countries to increase their output so that Russia cannot use its oil and gas wealth to "blackmail" other nations.
Biden saw Ukrainian Foreign Minister Dmytro Kuleba and Defence Minister Oleksii Reznikov in the Polish capital Warsaw in his first face-to-face meeting with top Ukrainian officials since the start of the war.
Biden described Putin as a "butcher" after touring a food kitchen for Ukrainian refugees.
"We are waiting for President Biden to close the sky over Ukraine. Ukrainian warriors can protect our country on the ground, but we can't close the sky," said Alla Dyachenko, a chemist from Kyiv who fled to Poland with her daughter.
Nato has ruled out a no-fly zone over Ukraine, fearing it would lead to direct clashes with Russian forces and a Europe-wide escalation.
Biden's visit to Poland was his final stop on a trip to Europe that has underscored his opposition to the Russian invasion, his solidarity with Ukraine and his determination to work closely with Western allies to confront the crisis.
Zelenskiy pushed late on Friday for further talks with Russia after its defence ministry said a first phase of its operation in Ukraine was mostly complete and that it would now focus on the Donbass region bordering Russia, which has pro-Moscow separatist enclaves.
Russian-backed forces there have been fighting pro-government forces since 2014.
Reframing Russia's goals may make it easier for Putin to claim a face-saving victory, analysts said.
Moscow has until now said its goals for what it calls its "special military operation" include demilitarising and "denazifying" its neighbour. Ukraine and its Western allies have called that a baseless pretext for an unprovoked invasion.
The United Nations has confirmed 1,081 civilian deaths and 1,707 injuries in Ukraine since the invasion but says the real toll is likely higher. Ukraine says 136 children have been killed.
Russia's defence ministry said 1,351 Russian soldiers had been killed and 3,825 wounded, the Interfax news agency reported on Friday. Ukraine says 15,000 Russian soldiers have been killed. Reuters could not independently verify the claims.
Footage from Mariupol, home to 400,000 people before the war, showed destroyed buildings, burnt out vehicles and shell-shocked survivors venturing out for provisions. Residents have buried victims in makeshift graves as the ground thaws.
"It's scary, I don't know how we're going to survive," an elderly woman resident said, declining to identify herself by name. "We're lying there, hoping they won't bomb us. Look at how many dead bodies we've buried around the building."
More than 100,000 people still need to be evacuated from the Mariupol, Deputy Prime Minister Iryna Vereshchuk said.
To the north, battle lines near the capital Kyiv have been frozen for weeks with two main Russian armoured columns stuck northwest and east of the city.
The Russian defence ministry said its troops had seized a dug-in command centre in a Kyiv suburb and captured more than 60 Ukrainian servicemen. Reuters could not immediately verify this.
A British intelligence report said Russian forces were relying on indiscriminate air and artillery bombardments rather than risk large-scale ground operations.
"It is likely Russia will continue to use its heavy firepower on urban areas as it looks to limit its own already considerable losses, at the cost of further civilian casualties," the latest British assessment said.
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