Russia reorganized the command of its flagging offensive in Ukraine, selecting a general with extensive combat experience in Syria to lead the mission, 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 an antiaircraft missile system, 800 antitank missiles and assorted armored vehicles to Ukraine, 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, flanked by the flags of both nations.

Mr. Johnson and Mr. Zelensky planned to discuss further support for Ukraine, including a “new package of financial and military aid,” the British government said in a statement.

The effort by Mr. Johnson and other Western leaders to bolster Ukraine came as fears of a new Russian onslaught escalated one day after a missile attack on a train station in the eastern city of Kramatorsk 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.

Mr. Zelensky described the attack as “another war crime of Russia” in his nightly videotaped address to the nation. He said the strike on innocent civilians at the station 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 military commanders to face trials like those faced by the Nazis at Nuremberg after World War II.

Mr. Zelensky thanked Ursula von der Leyen, the president of the European Commission, who visited Bucha on Friday, “for her personal involvement and assistance in setting up a joint investigation team to establish the full truth about the actions of the Russian occupiers and bring all those responsible to justice.”

Japan said it would join the United States and European nations in supporting investigations into what Prime Minister Fumio Kishida called “unforgivable war crimes” committed by Russian troops.

Mr. Kishida accused Russia of having repeatedly violated international humanitarian law by attacking civilians and nuclear power plants, a sore point for Japan given its 2011 experience with the nuclear disaster at the Fukushima Daiichi plant.

“We must hold Russia strictly accountable for these atrocities,” Mr. Kishida said. Japan said it would also expel eight Russian diplomats, ban Russian coal and restrict Russian imports of timber, vodka and machinery.

Legal experts have said that bringing war crimes charges against the Kremlin 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.”

On Friday, the day of the missile strike in Kramatorsk, more than 6,600 people managed to flee besieged Ukrainian cities — a record number for the week — the country’s deputy prime minister, Iryna Vereshchuk, said.

The mayor of Kramatorsk, Oleksandr Honcharenko, said that he expected about one-quarter of the city’s 200,000 residents to remain in the city, despite the expected Russian advance. He said the city 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 7 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 reorganization of the Russian military command 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.”

Britain’s Defense Ministry also pointed to Russian military challenges, even as it warned that Russia was expected to escalate its airstrikes in eastern and southern Ukraine. The ministry said that Russian efforts to link its soldiers in Crimea, which Russia annexed in 2014, with Kremlin-backed troops in the Donbas region were being thwarted by Ukrainian counterattacks.

The appointment of General Dvornikov, reported by a senior U.S. official on Saturday, was an effort to right that struggling campaign, American officials said.

General Dvornikov, 60, holds the second-highest rank in the Russian army. He was named a hero of the Russian Federation for his command of Russian forces in the brutal war in Syria, where Mr. Putin deployed Russian warplanes and missiles to help Syria’s president, Bashar al-Assad, in a multi-sided conflict among the government, armed rebels, jihadists and others. In September 2016, the general was appointed commander of Russia’s Southern Military District, with responsibility for the restive North Caucasus.

Russia had been running its military campaign against Ukraine out of Moscow, with no central war 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, and Steven Erlanger and Matina Stevis-Gridneff from Brussels.