Europe has faced to calls to step up its military support to Ukraine as US President Donald Trump has plunged Washington's backing into doubt.

Kallas floated a plan this month to get EU countries to commit to giving Kyiv from 20 to 40 billion euros in key weaponry in 2025.

But the initiative -- which was based on member states contributing according to their economic weight -- was opposed by countries including Italy, Spain and France.

"This is a long term thing that countries are not at this moment ready to agree," Kallas said Wednesday.

She said that instead Brussels was now working getting member states to agree on meeting a Ukrainian request to rush two million shells to Kyiv worth five billon euros.

"I think we can do it," she said.

"That is a very, very short term. Either you chip in with cash, or you come in with the ammunition."

Kallas was speaking ahead of the EU unveiling proposals on Wednesday on how to bolster its defences in the face of the menace from Moscow and doubts over US protections.

A key part of that plan also involved bolstering Ukraine in the short term.

The latest initiative from Kallas is set to be discussed by EU leaders at a summit in Brussels on Thursday.