BRUSSELS: The EU urged doubtful national leaders Friday to back Jean-Claude Juncker's new investment plan at a summit next week after the scheme struggled to garner enthusiasm from governments.

New European Commission head Juncker last month revealed a 315-billion-euro investment scheme to "kickstart" the EU's stalling economy, declaring it a major plank of his five-year agenda.

The EU's 28 leaders are meeting next week to endorse the plan, but Brussels is hoping for not only a greenlight but more funding as well.

"The commission expects that head of state and governments next week offer a strong and unequivocal sign of political endorsement," Juncker's spokesman Margaritis Schinas said at a news briefing.

The 315-billion-euro figure "is only an indicative target that we can overshoot with member states' contributions", he said.

The current estimate is based on 21 billion euros in seed money provided by the Commission and the European Investment Bank, money readily available to Juncker, which will act as leverage for further growth.

But with no control on how the money will be spent, and confusion on how the contributions will be counted, member states have so far held back from adding their pledges.

"We shouldn't expect a donor's conference," one diplomat from a major member state said in advance of next week's summit.

To drum up more funding, Juncker's plan proposes to effectively deduct contributions when the Commission assesses national budgets, part of the EU's new powers to oversee public spending by member states.

Schinas said it would be "budget neutral" — meaning that it would not appear as an increase in spending for member states.

But with cash strapped European governments under pressure from their voters about spending by Brussels, diplomats and officials said they were still unclear whether that was really the case.

"If that's not clear, no one is going to be interested," one senior government official told AFP.

Spanish Finance Minister Luis De Guindos said earlier this week that Madrid was "open" to participate but needed "more details".

Details would likely include more specifics on how the plan will allocate funds — and to what projects.

Despite their doubts, on Tuesday the member states unveiled a list of 2,000 projects worth 1.3 trillion euros.

The projects range from an airport expansion in Germany's Frankfurt, to a underwater fiber-optic link between the Canary Islands and Brazil.