Western allies need to “ramp up” arms production in the coming months to ensure Ukraine is kept fully supplied in its war against Russia, the secretary-general of Nato has said.

Jens Stoltenberg said the conflict is consuming an “enormous amount” of munitions, amid signs that President Vladimir Putin is digging in for the “long haul”.

He said Nato allies need to maintain their own stockpiles of weapons as well as continuing to supply the government in Kyiv with the armaments it needs.

He told BBC Radio 4’s The World This Weekend: “For the artillery, we need an enormous amount of ammunition, we need spare parts, we need maintenance.

“This is a huge undertaking. We need to ramp up production, and that is exactly what the Nato allies are doing.

“It is a core responsibility for Nato to ensure that we have the stocks, the supplies, the weapons in place to ensure our own deterrence and defence, but also to be able to continue to provide support to Ukraine for the long haul.”

His call came as Russia’s pummelling of civilian areas in Ukraine showed little sign of abating, with further missile strikes over the weekend on the capital, Kyiv, and on the eastern city of Kherson.

Mr Stoltenberg said that while the Ukrainians had enjoyed the upper hand in the fighting in recent weeks, there are indications the Russians are regrouping for a renewed offensive.

“Russia has shown no sign of giving up its overall goal of taking control over Ukraine,” he said.

“The Ukrainian forces have had the momentum for several months but we also know that Russia has mobilised many more forces. Many of them are now training.

“All that indicates that they are prepared to continue the war and also potentially try to launch a new offensive.”

While Mr Stoltenberg said he believes the war will “most likely” end around the negotiating table, he said it is essential to ensure the Ukrainians are able to enter any talks from a position of strength.

“What Ukraine can achieve round that table will depend on the strength on the battlefield,” he said.

“If we want a negotiated solution that ensures that Ukraine prevails as a sovereign, independent, democratic state in Europe, then we need to provide support for Ukraine now.”

Earlier, the prosecutor who led the case against Serb leader Slobodan Milosevic called for Mr Putin to be tried for war crimes.

Sir Geoffrey Nice, who worked with the International Criminal Tribunal for the former Yugoslavia, said the case against the Russian leader “couldn’t be clearer”.

He told BBC Radio 4’s Broadcasting House: “It is crimes against humanity because civilian targets should never be bombed or otherwise attacked.

“There can be no doubt about the chain of command leading directly to Putin. These are his soldiers. He’s a guilty man.”

Sir Geoffrey said he was surprised prosecutors and politicians are not stating this more openly and expressed concern that Mr Putin could be exempted from trial as part of a deal to end the war.

“It quite possibly is the case that there will be a settlement agreement drafted by someone or other, not by the Ukrainians, which will have a clause in it saying Putin will not be tried,” he said.

“That’s an appalling prospect and it will be a complete denial of justice to the people of Ukraine.”