European Commission President Ursula von der Leyen has travelled to Kyiv, leading a diplomatic delegation designed to send a "message of support" to Ukraine.
Von der Leyen was joined by EU foreign policy chief Josep Borrell and Slovakian Prime Minister Eduard Heger.
The trio travelled by train from neighboring Poland, departing the city of Przemsyl in a pre-dawn crossing of the border into the conflict zone.
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One of the first stops was to the city of Bucha, where the EU leaders were shown the bodies of civilians that Ukraine says were killed during Russia's occupation of the area.
"The unthinkable has happened here," von der Leyen told reporters.
"We have seen the cruel face of Putin's army, we have seen the recklessness and cold-heartedness with which they have been occupying this city. Here in Bucha, we saw our humanity being shattered. The whole world is mourning with the people of Bucha."
European Commission President Ursula von der Leyen, High Representative of the European Union for Foreign Affairs and Security Policy Josep Borrell, Slovakia's Prime Minister Eduard Heger and Ukraine's Prime Minister Denys Shmyhal visited the site of a mass grave in Bucha. /Valentyn Ogirenko/Reuters
European Commission President Ursula von der Leyen, High Representative of the European Union for Foreign Affairs and Security Policy Josep Borrell, Slovakia's Prime Minister Eduard Heger and Ukraine's Prime Minister Denys Shmyhal visited the site of a mass grave in Bucha. /Valentyn Ogirenko/Reuters
More sanctions
The cross-country train journey comes as the West looks to strengthen its support for Ukraine, with NATO signing off on the delivery of more advanced weaponry, and the EU voting for a fifth round of sanctions against Russia, targeting coal and high tech exports.
Slovakia also announced it has supplied the Ukrainian military with an S300 air defence system.
"I believe that this defence system will help save as many innocent lives as possible from the aggression of Putin's regime," Slovakian Prime Minister Eduard Heger said in a video message.
Donating this system however does not mean Slovakia has become part of an armed conflict. I want to assure all citizens of the Slovak Republic that protection of our territory is sufficiently secured and in the coming days will be strengthened by another system from our allies," he added.
UN sounds alarm over aid blockages
Von der Leyen's visit came as the United Nations said it was still struggling to get aid to civilians in the east of the country, where heavy fighting continued to take place.
"We try to push aid in there as much as we can. People are still hunkered down in basements in Luhansk and Donetsk," said Jens Laerke, from the UN's Office for the Coordination of Humanitarian Affairs (OCHA).
"We have in our planning, convoys to go there, I understand already next week. Everything again, whether that happens or not, depends on the security situation. But it will be ready to go there, if we can get in.”