German Vice Chancellor Robert Habeck on Wednesday criticised Switzerland for refusing to supply munitions for German-made Gepard anti-aircraft guns being used by Ukraine to repel Russia's attack.

"Some countries still have munitions but are reluctant to supply them to Ukraine for historical reasons," Habeck said in an interview with Die Zeit newspaper.

"We are in talks with Switzerland, and I have to be clear: I cannot understand why Switzerland does not provide Gepard munitions," he said.

The comments came as Nato allies were meeting in Brussels in a bid to speed up deliveries of munitions and arms to Kyiv.

Nato chief Jens Stoltenberg warned on Monday that Kyiv is using ammunition at a faster rate than Nato allies are producing it.

"The war in Ukraine is consuming an enormous amount of munitions, and depleting allied stockpiles," he said.

Berlin has sent German-made Gepard systems and munitions to Ukraine as part of a package of weapons to help the country repel Russia's invasion.

But Germany's own production of munitions is limited and it has asked Switzerland for authorisation to send Ukraine Swiss-made ammunition.

Switzerland has declined, reasoning that authorising the deal would compromise its neutrality.

German manufacturer Rheinmetall, which makes the Gepard, has said it will open a new domestic facility to make equipment for the anti-aircraft system, with production due to start in June.

German Defence Minister Boris Pistorius said on Tuesday that Berlin had signed a deal with Rheinmetall to produce munitions for the guns.

Rheinmetall confirmed on Wednesday that it has been contracted to supply 300,000 rounds of 35mm ammunition for the Gepard.

The order value is "in the low three-digit million euro range", it said. (AFP)