The European Union should be prepared to go as far as doubling its military aid to Ukraine this year to 40 billion euros ($43 billion) if necessary, according to a discussion paper by the bloc's diplomatic service seen by Reuters on Friday.

The paper, an updated version of an earlier proposal which set out Ukraine's requirements but did not name a target figure, also says each EU country participating in the effort should contribute "in line with its economic weight."

With the future of U.S. backing for Ukraine uncertain, EU countries have expressed a broad willingness to continue supporting the country in the war triggered by Russia's 2022 invasion, but diplomats say several states oppose fixed targets.

The paper says the EU gave about 20 billion euros in military aid to Ukraine last year and encourages EU countries to do at least the same again in 2025, with the total "potentially reaching" 40 billion euros, depending on Kyiv's needs.

EU leaders are likely to discuss the proposal by the European External Action Service (EEAS), headed by former Estonian Prime Minister Kaja Kallas, at a summit next week.

The EEAS "non-paper" - diplomatic jargon for a discussion document rather than a formal proposal - suggests leaders agree as a first step next week to provide 2 million rounds of large-caliber artillery ammunition to Kyiv this year.

It suggests that some of the costs of the plan could come from a scheme that takes windfall profits from Russian assets inside the EU and uses them for the benefit of Ukraine.

The contents of the new proposal were previously reported by Politico late on Thursday evening.

Hungary, whose prime minister Viktor Orban has criticized EU sanctions on Russia and military assistance to Ukraine, said on Friday his country should contribute funds to a common EU defense policy, but not agree to joint EU borrowing for defense. ($1 = 0.9227 euros)