The governor of Kyiv region said on Saturday that Russian forces had taken control of a town populated by workers from the defunct Chernobyl nuclear power plant, and fighting was reported in the streets of the besieged southern port of Mariupol.

Russia has failed to seize any major Ukrainian city after more than four weeks of conflict, and Moscow signaled on Friday that 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.

U.S. 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. r