Russia reorganized the command of its flagging offensive in Ukraine on Saturday, selecting for the mission a general accused of ordering strikes on civilian neighborhoods in Syria, as Western nations poured more weapons into the country in anticipation of a renewed Russian assault in the east.
The appointment of the general, Aleksandr V. Dvornikov, as the top battlefield commander came as Britain announced that it was sending missiles that target aircraft, tanks and even ships, and as Slovakia handed the Ukrainian military a long-range S-300 air defense system, with the blessing of the United States.
In another show of support for Ukraine, Prime Minister Boris Johnson of Britain made a surprise visit on Saturday to Kyiv, the capital, where he met with Ukraine’s president, Volodymyr Zelensky, and discussed a “new package of financial and military aid,” the British government said.
Mr. Zelensky called on other Western leaders to similarly provide military aide to Ukraine and impose further sanctions on Russia.
“Other Western democratic countries should follow the U.K.’s example,” Mr. Zelensky said after meeting with Mr. Johnson.
The two leaders walked through the mostly empty cobbled streets of Kyiv in a show of confidence that the Ukrainian capital was now safe from Russian attacks. Outside a shop, one man warmly greeted them, thanking Mr. Johnson for Britain’s support in effusive Ukrainian as Mr. Zelensky translated.
“In the last few weeks the world has found new heroes, and those heroes are the people of Ukraine,” said Mr. Johnson.
“What Putin has done in places like Bucha and Irpin, his war crimes, have permanently polluted his reputation and the reputation of his government,” he added. “There is a huge amount to do to make sure that Ukraine is successful, that Ukraine wins and that Putin fails.”
The effort by Mr. Johnson to bolster Ukraine came as fears of a new Russian onslaught escalated. Despite its large army and considerable military might, Russia was unable to take Kyiv and now appears to be scrambling to retain dominance in Ukraine’s southeast, appointing a new commander for its offensive and withdrawing troops from the capital to an area where it has the advantage of support from local ethnic Russian separatists.
“Russian forces continue to attempt to regroup and redeploy units withdrawn from northeastern Ukraine to support an offensive in eastern Ukraine, but these units are unlikely to enable a Russian breakthrough and face poor morale,” said a report from the Institute for the Study of War, a Washington think tank.
Even so, Russia’s air campaign and missiles continue to cause grave damage. A missile attack on a train station in the eastern city of Kramatorsk on Friday killed more than 50 people, including children, and injured many more who were heeding official warnings to flee.
Moscow denied responsibility for the attack, but U.S. military officials and independent analysts in Washington said they believed Russian forces had launched the missiles.
In a statement condemning the train station attack, the European Union said on Saturday that Russia was clearly culpable and that “attempts to hide Russia’s responsibility for this and other crimes using disinformation and media manipulations are unacceptable.”
Mr. Zelensky described the attack as “another war crime” and said it would be investigated, along with other atrocities attributed to Russian troops, including the apparent murders of civilians in Bucha, a suburb of Kyiv.
“Like the massacre in Bucha, like many other Russian war crimes, the missile strike on Kramatorsk must be one of the charges at the tribunal, which is bound to happen,” Mr. Zelensky said, calling for Russian commanders to face trials like those faced by the Nazis at Nuremberg after World War II.
Japan said it would join the United States and European nations in supporting investigations and would expel eight Russian diplomats, ban Russian coal and restrict Russian imports of timber, vodka and machinery.
Japan accused Russia of repeatedly attacking civilians and nuclear power plants, a sore point for Japan after the 2011 nuclear disaster at the Fukushima Daiichi plant.
“We must hold Russia strictly accountable for these atrocities,” the Japanese prime minister, Fumio Kishida, said.
Legal experts have said that bringing war crimes charges against Kremlin officials would be difficult. The burden of proof is very high, requiring prosecutors to show that soldiers and their commanders intended to violate the international law that establishes the rules of war.
Western analysts and European intelligence officials believe that President Vladimir V. Putin of Russia is trying to achieve battlefield gains by May 9, when he is planning to give a victory day speech commemorating both the Soviet victory in World War II and the military operation in Ukraine.
On Saturday, Russian forces stepped up shelling in eastern Ukraine, with explosions reported in the Odesa and Kharkiv regions. The massing of Russian forces in the region, after they withdrew from areas around Kyiv, has prompted officials in the east to urge residents to flee. And thousands have.
“The Russian troops are coming, so we are leaving to save our lives,” said Svitlana Kyrychenko, 47, who evacuated from Kramatorsk with her 18-year-old daughter, elderly mother and aunt on Saturday morning. She was at the train station in the central city of Dnipro, looking for a place to stay.
“I brought nothing with me,” she said. “I only brought my documents and clothes to change into for a few days.”
Elsewhere in Dnipro, dozens of people waited to board buses to Bulgaria.
“The air raids are becoming more and more frequent,” said Ludmila Abramova, 62, who had fled from Pavlograd, a city close to the eastern Donbas region, where Russia has been refocusing its forces. “I’m leaving.”
“But it’s all going to be all right,” Ms. Abramova added. “I’ll be back soon.”
More than 6,600 people managed to flee besieged Ukrainian cities on Friday — a record number for the week — according to the country’s deputy prime minister, Iryna Vereshchuk.
But in Kramatorsk, there was no sense of panic after the train station attack, said the mayor, Oleksandr Honcharenko. He said that he expected about one-quarter of the city’s 200,000 residents to stay there, and was preparing food, water and medical supplies.
“The only thing that will convince them to leave the city is if it comes under siege,” Mr. Honcharenko said.
Fewer than 400 people had boarded buses out of Kramatorsk on Saturday, he said, presumably headed for areas to the west that are believed to to be safer.
The European Commission on Saturday said that a global fund-raising effort called “Stand Up for Ukraine” had raised 9.1 billion euros, including 1 billion euros from the commission, for people fleeing the Russian invasion.
More than seven million Ukrainians have left their homes since the invasion on Feb. 24, and more than 4.4 million have left the country altogether, in the fastest-moving exodus of European refugees since World War II, according to the United Nations.
The appointment of General Dvornikov came as the Institute for the Study of War, a Washington think tank that tracks the fighting, said in its latest assessment that Russian forces in the east appeared to be stalled, and were “unlikely to enable a Russian breakthrough and face poor morale.”
General Dvornikov was the first commander dispatched by Moscow to oversee Russian forces in Syria’s civil war in 2015 after the Kremlin intervened to shore up President Bashar al-Assad’s struggling military.
General Dvornikov was there for about a year and was named a hero of the Russian Federation for his role. He oversaw forces that have been widely accused of bombing civilian neighborhoods, targeting hospitals and resorting to other scorched-earth tactics to break the back of the rebel movement that sought to oust Mr. al-Assad.
“Bashar al-Assad is not the only one to be held accountable for killing civilians in Syria. The Russian general should, too,” said Rami Abdulrahman, the head of the Syrian Observatory for Human Rights, a war monitor based in Britain. “As the commander of military operations, that means he’s behind killing Syrian civilians by giving the orders.”
The actions of the Syrian government and Russian forces were widely decried by Western officials and human rights organizations, which said that some of their tactics amounted to war crimes.
The commander of a Syrian Christian militia that received support from and fought alongside Russian forces in Syria said General Dvornikov was involved in battles in many parts of the country.
“He was a real commander, very serious, proud of the Russian army and its military history,” the commander said, speaking on condition of anonymity because he was not authorized to speak with journalists.
Russia had been running its military campaign against Ukraine out of Moscow, with no central commander on the ground to coordinate air, ground and sea units. That approach helped to explain why the invasion struggled against an unexpectedly stiff Ukrainian resistance, and was plagued by poor logistics and flagging morale, American officials said.
The disorganized assault also contributed to the deaths of at least seven Russian generals, as high-ranking officers were pushed to the front lines to untangle tactical problems that Western militaries would have left to more junior officers or senior enlisted personnel.
Eric Schmitt reported from Washington, Jane Arraf from Lviv, Ukraine, and Michael Levenson from New York. Reporting was contributed by Andrew Higgins in Kosice, Slovakia, Thomas Gibbons-Neff and Natalia Yermak from Dnipro, Ukraine, Cora Engelbrecht from Krakow, Victoria Kim from Seoul, Julian E. Barnes from Washington, Ben Hubbard and Hwaida Saad from Beirut and Steven Erlanger and Matina Stevis-Gridneff from Brussels.