NEW YORK (NYTIMES) - Russia reorganised the command of its flagging offensive in Ukraine on Saturday (April 10), selecting for the mission a general accused of ordering strikes on civilian neighbourhoods in Syria as Western nations poured more weapons into the country in anticipation of a renewed Russian assault in the east.

The appointment of General Alexander 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 defence system with the blessing of the United States.

In another show of support for Ukraine, British Prime Minister Boris Johnson made a surprise visit on Saturday to Kyiv, the capital, where he met Ukrainian 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 aid to Ukraine and impose further sanctions on Russia.

"Other Western democratic countries should follow the UK's example," Mr Zelensky said after meeting Mr Johnson.

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 south-east, 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"Russian forces continue to attempt to regroup and redeploy units withdrawn from north-eastern Ukraine to support an offensive in eastern Ukraine, but these units are unlikely to enable a Russian breakthrough and face poor morale,"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 that tracks the fighting.

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 US 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 in Nuremberg after World War II.

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 Russian President Vladimir Putin 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 Odessa 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.

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.

Gen 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.

The general 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 neighbourhoods, 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 Mr Rami Abdulrahman, head of the Syrian Observatory for Human Rights, a war monitor based in Britain. "As the commander of military operations, that means he is 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 organisations, 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 Gen 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 authorised to speak to 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, US officials said.

The disorganised 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.