Russia's invasion, which began on 24 February, has forced more than four million people to flee abroad, killed or injured thousands, left a quarter of the population homeless, and turned cities into rubble.
The civilian casualties have triggered a wave of international condemnation, in particular over the deaths in the town of Bucha, a town to the northwest of Kyiv that until last week was occupied by Russian forces.
Russia has denied targetting civilians in what it calls a "special operation" to demilitarise and "denazify" its southern neighbour. Ukraine and Western nations have dismissed this as a baseless pretext for war.
Nehammer visited Ukraine a day after European Commission President Ursula von der Leyen - visits aimed at underlining the West's support for Zelensky. In another such move, Italy said it would re-open its embassy in Kyiv after Easter.
52 killed at station
Friday's missile attack on the train station in Kramatorsk, a hub for civilians fleeing the east, left shreds of blood-stained clothes, toys and damaged luggage strewn across the station's platform.
City mayor Oleksander Honcharenko, who estimated 4,000 people were gathered there at the time, said on Saturday that the death toll had risen to least 52.
Russia's defence ministry denied responsibility, saying in a statement the missiles that struck the station were used only by Ukraine's military and that Russia's armed forces had no targets assigned in Kramatorsk on Friday.
Russian state television described the attack as a "bloody provocation" by Ukraine.
In Washington, a senior defence official said the United States did not accept the Russian denial and believed Russian forces had fired a short-range ballistic missile in the attack.
Reuters was unable to verify the details of attack.
Honcharenko said he expected just 50,000-60,000 of Kramatorsk's population of 220,000 population to remain within a week or two as people flee the violence.
The Ukrainian military says Moscow is preparing for a thrust to try to gain full control of the Donbas regions of Donetsk and Luhansk that have been partly held by Moscow-backed separatists since 2014.
Air attacks are likely to increase in the south and east as Russia seeks to establish a land bridge between Crimea - which Moscow annexed in 2014 - and the Donbas but Ukrainian forces are thwarting the advance, the British defence ministry said in an intelligence update.
Russia's military said on Saturday it had destroyed an ammunition depot at the Myrhorod Air Base in central-eastern Ukraine.
Foreign leaders visit
EU chief von der Leyen said on Saturday Russian forces appeared to have committed war crimes by targeting civilians in Ukraine, but she said lawyers must investigate the alleged incidents.
She said she had seen with her own eyes on Friday the destruction in the town of Bucha near Kyiv. A forensics team began exhuming a mass grave on Friday containing the bodies of civilians who local officials say were killed while Russians occupied the town.
"My instinct says: If this is not a war crime, what is a war crime, but I am a medical doctor by training and lawyers have to investigate carefully," von der Leyen told reporters on board a train leaving Ukraine.
The Kremlin has repeatedly rejected accusations it has committed war crimes and has called allegations that its forces executed civilians in Bucha a "monstrous forgery".
The visits by foreign leaders and Italy's announcement on Saturday that it intends to re-open its embassy in Kyiv later this month marked a fresh sign that the city is returning to some degree of normality after Russian forces pulled out of areas to the north of the capital just over a week ago.
Some Ukrainians have also begun returning to the capital, with cafes and restaurants reopening.
The EU on Friday overcame some divisions to adopt new sweeping sanctions against Russia, including bans on the import of coal, wood, chemicals and other products. Oil and gas imports from Russia so far remain untouched.
Zelensky urged the West on Friday to do more. On Saturday, he said he understood the sanctions could cause financial losses for the countries imposing them.
"Nevertheless, there are countries which aren't afraid of those important decisions. I am aware of Austria's support in this issue," he said, again calling for weapons from "our partners".