Von der Leyen was reacting to an overnight Russian assault on Kyiv and the wider region.
"Last night, the Russian regime once again blindly attacked civilians from the air, with over 400 drones and missiles attacking the capital," she wrote on X.
Ukrainian officials said at least 14 people were killed and nearly 80 injured in the strikes, among them children.
Von der Leyen said the EU continues to stand by Kyiv.
Last week the bloc paid out the first instalment of a EUR 90 billion loan to Ukraine – EUR 4 billion earmarked for strengthening the country's drone technology – with further funds expected soon.
She also confirmed that Brussels is working to finalise a 21st package of sanctions against Moscow in the coming days.
"We will keep increasing the pressure until Russia ends the bloodshed," she said.
Ukrainian President Volodymyr Zelensky also called on Sunday for Western allies to bolster Ukraine's air defences, particularly by supplying more interceptor missiles for US Patriot systems.
He warned that Russia was preparing a large-scale strike timed to precede the NATO summit, which takes place in Ankara, Turkey, on Tuesday and Wednesday.
(ał)
Source: PAP