President Joe Biden and other leaders from the G7 -- Britain, Canada, France, Germany, Italy, Japan and the United States -- will join Ukrainian President Volodymyr Zelensky at 3:10 pm in Vilnius (1210 GMT) to announce the plan, the White House said.

"It signals a joint long-term commitment to building a powerful defensive... force for Ukraine," lead White House advisor for European affairs Amanda Sloat told reporters.

"This multilateral declaration will send a significant signal to Russia that time is not on its side."

The United States and its allies have so far been pouring weaponry and economic aid into Ukraine on a temporary basis to help the pro-Western country fight off a Russian invasion launched in February last year.

With Ukraine facing a likely lengthy wait to achieve its dream of NATO membership -- thereby gaining the alliance's automatic protection -- the G7 countries see their plan as an interim guarantee of assistance.

Sloat, from the White House's National Security Council, told reporters that the G7 leaders, in Vilnius for the NATO summit, want to "help Ukraine build a military that can defend itself and deter a future attack."

There will be emphasis on "long-term investment" both in the military and economic spheres, she said.

However, beyond the group pledge, the details of the aid being offered have not yet been announced. Sloat said each country would enter its own bilateral talks with Ukraine on how to "make this a reality."

The G7 plan is being presented as reassurance for Ukraine that it will not be abandoned after NATO made clear in Vilnius that Kyiv's bid to join the alliance still faced considerable obstacles.

"There is still the need for Ukraine to take further democratic and security sector reforms," Sloat said. Biden "has been clear that we think Ukraine can get there, but that is still going to be a requirement for Ukraine to join as it has been, frankly, for all members who have joined the alliance."

In the United States, comparisons are drawn between the interim plan and the approximately $3.8 billion in military aid provided to Israel annually by Washington on a 10-year basis.

However, there remain questions over how much money the individual governments will agree to set aside for Ukraine and how successful they will be in ensuring that the aid survives domestic political change.

"The expectation is that those will last for the long term," Sloat said.