Speaking alongside Zelensky in Brussels, von der Leyen said Kyiv must obtain credible security guarantees from Washington.
She also announced the EU was preparing another package of sanctions against Russia to be adopted in September.
"As long as the bloodshed in Ukraine continues, Europe will maintain diplomatic and, in particular, economic pressure on Russia," von der Leyen said.
She added that frozen Russian assets had already been used to support Ukraine.
Zelensky dismissed any proposals of Russian security guarantees, referring to comments made by Russian President Vladimir Putin.
He said that true security guarantees meant a strong Ukrainian army, financially supported by Europe, supplied with European weapons and supplemented with arms from the United States.
Any guarantee "must really be very practical, delivering protection on land, in the air, and at sea, and must be developed with Europe's participation," the Ukrainian president added.
The Sunday meeting in Brussels between Zelensky, von der Leyen and several European leaders came before Monday’s Washington talks, which French President Emmanuel Macron said were aimed at showing a "united front" between Ukraine and its allies.
"If we show weakness today in front of Russia, we are laying the ground for future conflicts," Macron said on Sunday.
Polish foreign ministry spokesperson Pawel Wroński stressed that a ceasefire was a prerequisite for peace talks.
"Any form of suspension, ceasefire, or demarcation of fire is, according to the Coalition of the Willing, a condition for starting any peace talks," he told reporters
European Council President Antonio Costa echoed the need for transatlantic unity.
"As I underlined during today's meeting of the Coalition of the Willing, if no ceasefire is agreed, the EU and the U.S. must increase pressure on Russia," Costa said in a message on X.
Following the Sunday talks, Polish Foreign Minister Radosław Sikorski added that "for peace to come, pressure must be placed on the aggressor, not the victim."
(ał)
Source: Reuters, IAR, PAP