The US will utilise $6bn (£4.8bn) for this purpose, Defence Secretary Lloyd Austin revealed on Friday.

Patriot air defence batteries are not included.

Ukrainian President Volodymyr Zelensky said Patriots were "urgently" needed to face a growing Russian air threat and "can and should save lives right now".

A source confirmed to the BBC that the $6bn was part of a $60bn aid package signed into law by US President Joe Biden on Wednesday, which also includes $1bn in more immediate aid.

Mr Austin told a news conference that the US was committing to its largest security assistance package to date and would "move immediately" to get the supplies to Ukraine.

These would include air defence munitions, counter-drone systems and artillery ammunition but not Patriots.

"It's not just Patriots that they [the Ukrainians] need, they need other types of systems and interceptors as well," Mr Austin said. "I would caution us all in terms of making Patriot the silver bullet."

He added that he was confident that more of the missile systems would be made available for Kyiv soon. Conversations were ongoing with European partners, he said, to deliver additional capabilities.

The Chairman of the US Joint Chiefs of Staff, General Charles Brown, said the assistance should eliminate the Ukrainians' need to ration shells on the frontline.

Some of the latest funding would also go to building up Ukraine's own defence industry, so that it can start manufacturing more of the ammunition it desperately needs.

Mr Austin said Russia had already increased domestic production of artillery ammunition and other weapons - as well as being propped up by supplies from Iran and North Korea.

"Understand what's at stake for Ukraine, for Europe, and for the United States," he said. "If Putin prevails in Ukraine - Europe would face a security threat it hasn't seen in a lifetime. Russia will not stop in Ukraine."

Asked whether the US aid would protect Ukrainian forces, Mr Austin said that the commitment was "material, real, and substantial" although "not instantaneous".

"It's going to take some time to get it in there and distribute. The Ukrainians were able to hold - with this capability, they can do a lot better."

The defence secretary's words come as Ukraine warned on Friday that Russia was ramping up attacks on its railways ahead of a fresh offensive.

A Ukrainian security source told the AFP news agency that Moscow wanted to damage Ukrainian railway infrastructure to "paralyse deliveries and movement of military cargo".

Ukraine only has a handful of Patriots to complement other Western missile defence systems and existing stocks of Soviet-era surface-to-air missiles (SAMs), such as the S-300.

They are the most capable and expensive air defence systems that Ukraine has. Each Patriot battery costs around $1bn (£800m), and each missile costs nearly $4m.

Germany has already promised an extra Patriot system - and its defence and foreign ministers appealed to their European counterparts earlier this month to respond urgently.

Greece has stocks of Patriots and S-300s but said none could be spared.

"We explained why we cannot do it," Greek Prime Minister Kyrios Mitsotakis told Skai TV.

His said his country's air defences were "critical systems for the protection of Greek air space".

According to reports, Spain will supply some Patriot missiles but not a full system.

Recent months have seen Kyiv step up its calls for Western assistance as its stocks of ammunition are depleted and Russia makes steady gains.

Ukrainian officials have blamed delays in military aid from the US and other Western allies for the loss of lives and territory.

By Holly Honderich & Will Vernon