On a tour of European capitals this week he received pledges from British Prime Minister Rishi Sunak and Dutch PM Mark Rutte to help build a "jets coalition", although both leaders have stopped short of saying they will send planes.

"We want to create this jets coalition and I'm very positive with it... I see that in the closest time you will hear some, I think, very important decisions, but we have to work a little bit more on it," Zelenskiy said in London on Monday.

Western governments have been wary of leaving their own countries undefended by giving away too much equipment. They have also avoided sending anything that could strike deep into Russian territory and give Moscow a reason to escalate the war.

WHAT KIND OF FIGHTER JETS COULD UKRAINE GET?

Ukraine wants F-16s, which it says are "four or five times" more effective than Soviet-era jets it currently has.

Washington has ruled out sending F-16s to Ukraine for now.

Any coalition of donors of the F-16, built by Lockheed-Martin, would likely be reliant on backing from the United States, by far the largest operator of the planes.

WHAT HAS BEEN GIVEN SO FAR?

No Western-designed jets have yet been donated. Poland and Slovakia have handed over 27 MiG-29s to supplement Ukraine's current fleet.

Poland has given Ukraine 14 MiG-29s and has said it will give more. However, Polish President Andrzej Duda reiterated on Tuesday that Poland has too few F-16 jets to be able to give any to Ukraine.

Slovakia has donated 13 Mig-29s in various states of airworthiness to Ukraine.

WHERE MIGHT THE JETS COME FROM?

Rutte said on May 4 that the Netherlands was working with allies including Britain, Belgium and Denmark to reach a conclusion on whether to send jets to Ukraine. The Netherlands has 24 F-16s operational until mid-2024, when they will be replaced by F-35s. It has also 18 F-16s non-operational, of which 12 have been sold.

Britain will not send jets to Ukraine, a spokesperson for Sunak has said, since Britain does not have the F-16s that Ukraine wants. Britain had previously said that the time required to train pilots and the substantial support crews needed to send its Eurofighter Typhoon jets meant they would be of little immediate use, but Sunak has said Britain stands willing to support any country willing to send jets.

German Defence Minister Boris Pistorius said on Wednesday the country has neither the training capacity nor the right equipment to actively contribute to the initiative to help Ukraine source fighter jets.

Denmark said in February it was "open" to the idea of sending fighter jets to Ukraine. Denmark currently has around 30 F-16s in operation.

One alternative to the F-16 could be the JAS Gripen fighter jet produced by Swedish defence manufacturer SAAB (SAABb.ST), which is seen as a more cost-effective alternative to the F-16. The aircraft is operated by Sweden and a handful of other countries such as South Africa and Brazil.

Sweden has repeatedly said there are no plans to send Gripens to Ukraine and that the Nordic country, which has applied to join NATO, needs its existing inventory. The Czech Republic uses rented Gripens while Slovakia is waiting to receive F-16s and currently relies on allies to cover its airspace.

Czech President Petr Pavel has mentioned that the Czech Republic could give some of its light, sub-sonic L-159s, but this is theoretical at this point.

HOW SOON COULD UKRAINE START USING THEM?

U.S. officials have estimated the most expeditious time needed for training and delivery of F-16s at 18 months.

French President Emmanuel Macron said on Monday that France is open to training Ukrainian pilots in France right away. France does not have F-16s, only French-made Rafale warplanes and the previous generation Mirage 2000 jets.

London has agreed to start training pilots in the spring and said it will look at shortening the sessions for experienced Ukrainian pilots.

“This is not about gifting weapon systems. This is about gifting a platform. If anyone follows Formula One, you don’t just gift a car, you have to gift a pit team," UK Defence Secretary Ben Wallace said on Wednesday.

Some fighter jet models are likely to prove more suitable than others.

Justin Bronk from the RUSI think tank said Typhoons and F-16s would need to operate from smooth runways and centralised bases, while Sweden's Gripen planes could fly at lower altitudes and be serviced on shorter, rougher airstrips.