Ukraine's President Volodymyr Zelensky spent much of the past week touring European capitals, appealing to leaders to send his country fighter jets.
At the start of Russia's full-scale invasion, Ukraine was believed to have around 120 combat capable aircraft - mainly consisting of aging Soviet-era MiG-29s and Su-27s.
But officials say they need up to 200 jets to match Moscow's air-power - which is thought to be five or six times greater than Kyiv's.
Mr Zelensky is primarily seeking US-made F-16s. First built in the 1970s, the jet can travel at twice the speed of sound and can engage targets in the air or on the ground.
While now eclipsed by the more modern F-35, it remains widely in use. Experts say modern fighters like the F-16 would help Ukraine strike behind Russian lines.
US President Joe Biden has ruled out supplying the jets for now. But countries like Poland and the Netherlands have signalled an openness to supplying Ukraine from their own fleet.
However, Mr Zelensky's request poses a host of practical challenges, that could make an early delivery of such aircraft unlikely. Here's four of them:
Long training times
Ukraine is said to have identified 50 pilots who could begin training on Western jets immediately, but preparing them to fly the warplanes takes time and takes them away from the current fighting.
The British government has agreed to start training Ukrainian pilots on Nato-standard aircraft, but warns that supplying the jets would only ever be a long-term option.
Training pilots could take months or even years, given how complex the fighter jets are. But British officials have said they could speed up the training process for some of Ukraine's more seasoned pilots, who have years of experience flying Soviet-era planes.
Maintenance issues
A fighter jet comes with an entire eco-system to support it. To function in a warzone, it needs complex and specialised engineering - it isn't "a simple case of towing an aircraft to the border," in the words of the UK's Defence Secretary Ben Wallace.
Former Nato official Dr Jamie Shea told the BBC that fourth-generation fighter jets - like the ones requested by Ukraine - require extensive maintenance after almost every flight.
"When I was at Nato and visited an airbase, planes would come back in and the engineers would have to strip out whole systems and put them back in. It's almost like every time you drive your car you have to put a whole carburettor in," Dr Shea said. "So they've got a very high maintenance requirement."
German MP Tobias Bacherle is also concerned that handing over the jets would be a long-term commitment, explaining that the issue is "not only about delivering".
"It's an ongoing engagement, this would be very different to delivering tanks, or delivering anti-aircraft missiles or other heavy weaponry as we've done before", he told the BBC.