If the EU tried to defend itself without help from NATO ally US, "it is estimated that an investment of several hundred billion euros would be needed to overcome the current capabilities gap," it underlined in a report.

Europe, particularly France, has increasingly been looking at creating a common defence capability with "strategic autonomy" to accompany its economic weight on the world stage.

That is why, for the bloc's 2021-2027 budget cycle, it intends to boost defence spending 700 percent, from 2.8 billion euros in the current multiyear cycle.

But there are concerns, also highlighted in the report, that the initiative would overlap with a lot of what NATO does. All but five of the European Union's 28 member states are in NATO.

The report noted that EU countries do not share a common threat perception, or vision of what exactly defence should consist of, or even rules of engagement.

Some member states, for instance, "tend to focus on territorial defence against the military threats posed by Russia, while others are more oriented towards security challenges originating in North Africa or the Middle East".

As a result, the EU's approach to defence was "vague".

- 'Duplication of NATO' -

Juhan Parts, a former Estonian premier who is a member of the European Court of Auditors that is tasked with scrutinising EU spending, called the mismatch between the ambitions and the resources on offer "huge".

He told AFP that "full-scale military planning (by the EU) is a duplication of NATO", but noted that was an issue for politicians, not for auditors.

Nevertheless, he said, the fact that NATO member Britain -- currently the EU's main national military power along with France -- was about to leave the bloc should give pause to those looking to bridge the gap between spending and strategic goals.

"If a very big member with a huge military budget and forces leaves the EU, then this gap becomes even bigger," he said.

"If we are now talking about 'strategic autonomy' and this mismatch (with resources) then, if the biggest (military) spender will leave, strategic autonomy becomes even more unrealistic," he said.