President Volodymyr Zelensky has knuckled down on his plea for Western aircraft after securing commitments for tanks, air defence and precision missiles.

But allies insist they are scrambling to ensure his forces have the ammunition they need on the ground to push back renewed Russian offensives.

Ukrainian Defence Minister Oleksiy Reznikov said he was focused on building an "aviation platform", winning more promises of tank supplies and ensuring ammunition stocks.

He was speaking at the meeting with more than 50 nations backing Kyiv at NATO headquarters in Brussels.

"This has become a grinding war of attrition and therefore it's also a battle of logistics," NATO Secretary General Jens Stoltenberg said.

"This is a huge effort by allies to actually be able to get in the ammunition, the fuel, the spare parts, which are needed."

Ukraine's Western supporters -- spearheaded by the United States -- have already supplied billions of dollars of arms to help Kyiv hold Moscow back.

- 'Fighting has intensified'-

Now, just under a year into the war, Stoltenberg said Russian President Vladimir Putin appears to have started a long-awaited new offensive in east Ukraine.

"Fighting has intensified and that means that our support for Ukraine also has to intensify," Dutch Defence Minister Kajsa Ollongren said.

The fighting is consuming vast quantities of ammunition, straining stockpiles and industries on both sides of the confrontation.

Stoltenberg warned that Kyiv's current rate of expenditure was "many times higher" than the output in NATO countries.

Allies continue to raid their shelves for the rounds -- especially 155-millimetre shells -- that Ukraine is firing by the thousands each day.

NATO is scrambling to get its factories to pump out more, and allies are eyeing plans for joint weapons purchases, higher defence spending and longer-term contracts.

Estonia has called for joint purchasing of arms by European Union countries to bulk buy weapons for Ukraine.

- Jet not 'must urgent' -

Zelensky issued a powerful call during a trip to London, Paris and Brussels last week for NATO members to send fighter planes and longer-range missiles.

The Ukrainian leader won a commitment from Britain to train pilots but did not get any firm promises that his forces will get Western planes.

"We know that Ukraine needs any kind of help and that means also the jet fighters," Estonian Defence Minister Hanno Pevkur said.

Slovakia has said it is willing to discuss sending Soviet MIG-29 planes to help replace losses to Ukraine's current stocks.

Dutch minister Ollongren said Kyiv had requested US-made F-16 jets from the Netherlands.

"We have to debate this with our partners, also with the United States, and we have to think about feasibility," she said.

"It is something that will take time and it is best done behind closed doors."

Stoltenberg insisted "the issue of aircraft is not the most urgent issue now, but it is an ongoing discussion".

"The urgent need now is to deliver what has already been promised, to deliver the armoured vehicles, the infantry fighting," he said.

Ukrainian troops have started training on German-made Leopard 2 battle tanks in Europe but there are concerns some nations that have pledged supplies are now stalling on sending them.