European Union countries are pushing to ramp up their defence industries and rearm their militaries in the face of Russia's invasion of Ukraine, after years of underinvestment.

"We estimate that additional defence investments of around 500 billion euros are needed over the next decade," von der Leyen said.

The head of the EU's executive did not, however, provide a detailed breakdown of what her projected sum covered.

"We didn't see spreadsheets, we didn't see details, this is pie in the sky money," an EU diplomat said.

There is also no clarity as yet on how the EU would finance the investment -- with options being mooted including national spending or joint EU defence bonds.

There is a standoff over whether the EU should consider joint borrowing, similar to how it financed its recovery programme after the Covid pandemic.

"Several countries, including France and Estonia, are in favour of eurobonds," an EU official said. "But Germany and the Netherlands are against."

"None of these options are easy, but all of them have to be looked at with the political will to decide what to do together, but they have to be looked at seriously," von der Leyen said.

EU countries have already increased their defence spending in the decade since Russia seized Crimea from Ukraine in 2014.

That trend has accelerated since the invasion of Ukraine in 2022, with a sharp increase in the number of EU countries hitting NATO's target of spending two percent of GDP on defence.

The bloc's defence agency says EU countries spent a total of 240 billion euros on defence in 2022.

The increases in EU defence spending in recent years have nonetheless been dwarfed by those of China and Russia.

"If you look at the combined EU spending on defence from 1999 to 2021 so in three years, it increased by 20 percent in that same timeframe, China's defence spending increased by almost 600 percent and Russia's defence spending by almost 300 percent," said von der Leyen.

"This is even before Russia massively increased over the last two years its defence spending."

As part of its push to bolster defence industries across the bloc, the EU is looking to appoint commissioner dedicated to working on the issue.