President Emmanuel Macron said that France would supply Mirage 2000-5 fighters, which will join the F-16 and Russian-made MIG-29 and Sukhoi models already in service.

"The F-16 and Mirage integration dilemma is part of a longer trend for Ukraine's armed forces since 2022... Ukraine has been struggling with integrating and adapting its newly received defence systems," said Ivan Klyszcz of the Estonia-based International Centre for Defence and Security.

There are many militaries around the world that operate different models of fighter plane.

But Kyiv's challenge is all the greater as it takes on new systems in the heat of Russia's two-year invasion, as well as combining modern Western kit with Soviet-era equipment.

"Ukraine's military engineers have been agile and highly skilled at adapting and integrating new systems," Klyszcz said.

President Volodymyr Zelensky has for months placed his country's need for improved air defences front and centre in conversations with allies.

He hopes an upgraded air force will be able to push back enemy artillery from the front lines and intercept more of the missiles and drones Russia uses to bombard Ukrainian territory.

"Most Russian airstrikes are launched from Russian territory," said Pierre Razoux of the Mediterranean Foundation for Strategic Studies.

"With (Western) authorisation to fire into Russia, the Ukrainians may be able to push back the safe distance at which Russian aircraft can safely fire" their weapons, he added.

- 'Few restrictions' -

A "coalition" of 11 countries was formed in July last year to train Ukrainian pilots on the F-16, with Britain, Canada and European countries including France joining in.

Denmark and the Netherlands have committed to delivering 61 planes after getting the go-ahead from Washington.

Norway and Belgium have since also pledged fighters.

Zelensky told AFP last month that Ukraine needs up to 130 F-16s to achieve parity with Russian aviation.

With some countries in Kyiv's coalition of backers more or less reluctant to allow their weapons to be used against Russian soil, having varied suppliers could be beneficial for Ukraine, said James Rands of British private intelligence firm Janes.

"France has been and is increasingly bullish about how weapons it donates should be employed," he told AFP.

"Therefore, Mirage is likely to arrive with few limitations on its employment."

Questions remain about the timing of the French support, after Macron set an ambition of five to six months.

In the case of the F-16, Ukraine's first pilot "probably" began training on the jets in August, Rands said.

"We think the first F-16s will be arriving in the next month or two," he added -- some ten months later.

- 'Out of step' -

Macron and Paris's defence ministry have yet to say how many aircraft France will provide.

Greece has a number of Mirage 2000-5 jets and could give some of them up.

But "for them to have an operational effect, you'd need at least 30 to 40, or two squadrons," Razoux said.

Leo Peria-Peigne of France's Institute for International Relations suggested that Ukraine should be given 20 Mirage at most.

"F-16s do air-to-ground (attack), the Mirages only do air-to-air, so they're quite complementary," he pointed out.

Nevertheless, France is "a bit out of step. There was a desire among NATO countries to combine their efforts around the F-16. I'm struggling to understand the logic" of Macron's announcement, he added.

In a similar case, Sweden decided against delivering its Gripen fighter jets as allies pushed to prioritise the F-16.

That leaves Paris out on a limb compared with its partners.

"The timing is questionable," said Justin Bronk of British defence think-tank RUSI.

"Gripen is more applicable to Ukraine operational requirements than Mirage 2000 and a lot more work has been done to train Ukrainian pilots on Gripen," he added.

Even if the French-built fighters are "probably an unhelpful distraction of resources at this stage," Zelensky "is not politically in a position to say no," Brink said.