NATO foreign ministers will hold preliminary talks on the plan in Brussels Wednesday as they seek to forge a support package for Ukraine by a July summit in Washington.

"Foreign ministers will discuss the best way to organise NATO's support for Ukraine, to make it more powerful, predictable and enduring," a NATO official said.

"No final decisions are to be taken at the April ministerial meetings, and discussions will continue as we approach the Washington summit in July."

Officials and diplomats said the proposal was for NATO's 32 countries to contribute to the fund according to the size of their economy.

But several cautioned that there remain major questions over how any financing would work and the plan would likely change markedly by the summit in Washington.

"There is still a long way to go -- numerous allies have questions on practical arrangements," a NATO diplomat said, speaking on condition of anonymity.

The officials said Stoltenberg's proposal also envisions NATO taking more control of coordinating arms supplies to Kyiv from a US-led grouping that currently helps oversee support.

Stoltenberg has argued this could help insulate the flow of weaponry to Ukraine from any political changes in NATO countries, with Donald Trump pushing to return to White House at November elections, officials said.

The move would mark a major shift for the Western military alliance which has so far refused as an organisation to send weapons to Ukraine for fear it would drag NATO closer to a conflict with Russia.

Up until now NATO has only sent non-lethal aid to Ukraine, while its individual members have supplied weaponry worth tens of billions of dollars.

The proposal comes as Ukraine's outgunned forces are struggling to hold back Russia in the face of dwindling supplies from Kyiv's Western backers.

A $60-billion US funding package is currently stalled in Congress but there are hopes lawmakers could move to pass it in the coming weeks.

NATO foreign ministers are also expected to discuss the race to replace Stoltenberg after Romanian President Klaus Iohannis launched a surprise challenge against the frontrunner, Dutch premier Mark Rutte.

Diplomats said Rutte now has the support of some 90 percent of NATO countries, but Hungary and Turkey remain holdouts blocking a swift nomination ahead of the summit.