LVIV, Ukraine (Reuters) – Rockets struck the western Ukrainian city of Lviv on Saturday, signalling a potential new front in Moscow’s invasion as U.S. President Joe Biden decried Russian President Vladimir Putin’s power and sought to steel Europe for a long fight ahead.

Intense fighting raged in several parts of Ukraine, suggesting there will be no swift let-up in the month-old war while Biden framed the fight as part of the historic struggle for democratic freedoms in a major address from Poland as he concluded his European trip aimed at bolstering Western resolve.

“For God’s sake, this man cannot remain in power,” Biden said in Warsaw. A White House official later said Biden was not calling for regime change but was saying “Putin cannot be allowed to exercise power over his neighbors or the region.”

The Kremlin dismissed Biden’s comment, saying it was “not for Biden to decide.”

After more than four weeks of fighting, Russia has failed to seize any major Ukrainian city and the conflict 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.

Moscow signalled on Friday it was scaling back its military ambitions to focus on territory claimed by Russian-backed separatists in the east of Ukraine.

But on Saturday, four rockets hit the outskirts Lviv, some 60 km (40 miles) from the Polish border for what appeared to be the first time since Moscow’s invasion. The western Ukrainian city had so far escaped the heavy bombardment and fighting that has devastated other cities closer to Russia.

Regional Governor Maksym Kozytskyy said five people had been wounded and residents were told to head to shelters after an initial strike hit mid-afternoon. Reuters witnesses saw black smoke rising from the northeast side of the city and Lviv’s mayor said an oil storage facility had been hit.

Ukrainian officials later reported another strike significantly damaged Lviv’s infrastructure but that so far there were no reported deaths.

Russian forces also seized Slavutych, a town where workers at the nearby defunct Chernobyl nuclear plant live, and three people were killed, Interfax Ukraine news agency quoted the local mayor as saying.

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, and the International Atomic Energy Agency (IAEA) has expressed alarm about the situation if workers are unable to rotate.

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, saying it would “deal a blow to countries worldwide.”

Speaking via video link, he also urged energy-producing countries to increase their output so that Russia cannot use its oil and gas wealth to “blackmail” other nations.

Zelenskiy in a call with Polish counterpart Andrzej Duda on Saturday also expressed disappointment that Soviet-made fighter aircraft in Eastern Europe had not been transferred to Ukraine, his office said in a statement.

Washington had rejected a surprise offer by Poland to transfer Russian-made MiG-29 fighter jets to a U.S. base in Germany to replenish Ukraine’s air force.

The United States, which has already pledged billions in aid, will give $100 million in civilian security assistance such as communication and field gear for Ukraine’s border guard and police, U.S. Secretary of State Antony Blinken said on Saturday.

Biden earlier on Saturday called Putin a “butcher” after meeting Ukrainian refugees at a food kitchen in Poland — a comment the Kremlin was cited by Russia’s TASS news agency as saying would further damage prospects for mending Russian-U.S. ties.

Biden also 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’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.

“We need to be clear-eyed. This battle will not be won in days or months … We need to steer ourselves for a long fight ahead,” Biden said in his speech.He also urged Europe to accelerate the shift to cleaner renewable energy and wean itself off Russian oil and gas.

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 demilitarizing and “denazifying” its neighbour. Ukraine and its Western allies have called that a baseless pretext for an unprovoked invasion.

The United Nations has confirmed 1,104 civilian deaths and 1,754 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.”

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.

The general staff of the Ukrainian armed forces, in a post, said Russia’s losses were such that Moscow was conducting “hidden mobilizations” and taking battle tanks out of long-term storage.

A British intelligence report said Russian forces were relying on indiscriminate air and artillery bombardments rather than risk large-scale ground operations, a tactic the report said could limit Russian military casualties but would harm more civilians in Ukraine.