Ukraine's President Volodymyr Zelensky attended a summit of some 50 European countries in Denmark / AFP

Ukraine's President Volodymyr Zelensky warned Europe on Thursday that recent drone incursions showed Russia was looking to "escalate" its aggression, as he offered his country's war-honed expertise to help counter the threat.

European leaders from just under 50 countries converged on a conference centre in Copenhagen under tight security after mystery drone flights rattled Denmark.

The drone sightings in Denmark and high-profile aerial incursions by Moscow in Estonia and Poland have heightened fears that Russia's assault on Ukraine could spill over Europe's borders.

"The recent drone incidents across Europe are a clear sign that Russia still feels bold enough to escalate this war," Zelensky said.

"It was never just about Ukraine, Russia has always aimed to break the West and Europe."

European leaders are keen to work with Ukraine's war-tested expertise as they seek to bolster their own defences, and are discussing plans for a "drone wall" to counter the menace from Moscow.

"If the Russians dare to launch drones against Poland, or violate the airspace of northern European countries, it means this can happen anywhere," Zelensky said.

"We are ready to share this experience with our partners."

French President Emmanuel Macron said that Western countries should be willing to take a tougher line when confronted by Russian drones, to sow doubt in the Kremlin.  

"It's very important to have a clear message. Drones which would violate our territories are just taking a big risk. They can be destroyed, full stop," Macron said.

Romanian Prime Minister Nicosur Dan, whose country has seen Russian drones crossing over from Ukraine, warned his forces would shoot down the next one to violate their airspace. 

- 'Kill' Russia's 'shadow fleet' -

As Russia's full-scale war on Ukraine drags on through a fourth year, Europe is scrambling to keep up pressure on Moscow and secure funding for Kyiv.

Macron said it was key to step up efforts to counter the so-called shadow fleet of ageing oil tankers to "kill" the business model Russia uses to circumvent restrictions on exporting its oil.

"It is extremely important to increase the pressure on this shadow fleet, because it will clearly reduce the capacity to finance this war effort," said Macron -- pointing at France's move this week to hold a blacklisted tanker linked to Russia.

In a bid to ensure Ukraine has the financing it needs, the EU is exploring a proposal to use frozen Russian assets to fund a new 140-billion-euro ($165-billion) loan. 

Proponents say that move is needed to help Ukraine plug budget shortfalls -- and that Russia, not European taxpayers, should ultimately foot the bill. 

But Belgium, where the vast majority of frozen assets are held, still has a lot of reservations over the plan.

"We're going to move to uncharted waters. This is very, very risky," Belgian Prime Minister Bart De Wever said. 

He insisted he wanted clear commitments from all EU leaders that they would share the potential liability with Belgium to shield it from any Russian retribution. 

EU chief Ursula von der Leyen said Wednesday it was clear that risk should not fall only on Belgium's shoulders and that she would "intensify" talks on the proposal.

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