European Union leaders have met for an informal gathering in Denmark for face-to-face meetings on a range of issues before a formal EU summit later on this month.

Wednesday's summit's main aim was to debate Europe's common defence and ongoing support for Ukraine.

The gathering of prime ministers, presidents and chancellors in Copenhagen addressed the suggestion of a so-called 'drone wall' which could be constructed in eastern EU member states. The wall would have the ability to track and intercept unauthorized incoming drones and other airborne objects.

But the financing and management of such a system remains unclear, with Germany and Hungary cooling their interest in the proposal, even as NATO boss Mark Rutte described it as "timely and necessary".

European leaders have been alarmed by the rise in the number of incidents involving remote flying objects, after a range of violations of EU and NATO airspace in recent weeks - including drone flights in Denmark.

An unidentified flying object was spotted over northern Denmark in the lead-up to the summit in Copenhagen.

The Danish capital's airport had to close last week due to drone sightings, and drones have also recently been seen in Poland and Romania.

European leaders suspect Russia is involved but Moscow says the accusations are baseless.

Mikkel Runge Olesen, senior researcher at the Danish Institute for International Studies, says the recent incursions identified some weak points in the country's defense of its airspace.

"Denmark and the Danish government has actually acknowledged that the capacity to fight off drones has not been sufficient at the moment and for that reason Denmark is seeking to make quick investments in order to fix that gap in Danish capabilities," he said.

Leaders also discussed a new package of sanctions to be imposed on Moscow, and addressed an idea to use frozen Russian assets to support Ukraine.

Defense and security were not the only topics on the agenda. On the sidelines, leaders said they wanted to remove barriers to competitiveness.

"We want to be more competitive because this is the only way to have growth, to create jobs, to finance our social model and the future of young people," said Emmanuel Macron, the French president.

But the conflict in Ukraine remained the most pressing issue, and the Danish capital will host more regional leaders for a wider summit on October 2, including the Ukrainian president Volodymyr Zelenskyy.