Former Ukrainian defence minister Mykhailo Fedorov has called for wartime elections, in what is widely seen as the biggest challenge to Volodymyr Zelensky's presidency since Russia's full-scale invasion four and a half years ago.

"Democracy cannot be held hostage by Russia," said Fedorov, 35, in a video posted to YouTube on Tuesday, adding that Ukraine is fighting "precisely because we want to remain a free European state".

Since the war broke out in 2022 Ukraine has been under martial law, which means elections are suspended.

Fedorov, who did not mention Zelensky by name in the video, was unexpectedly fired from his role as defence minister last month, just six months after his appointment.

Zelensky is yet to respond to the video from Fedorov, whose dismissal sparked protests across the country.

During his tenure as a highly popular defence minister, Fedorov was credited with energising the ministry, heading a drive against corruption and using data to analyse and try to improve performance on the front line.

His dismissal is widely accepted to have been related to tensions between him and army commander-in-chief, Oleksandr Syrskyi.

Within days Syrskyi was also replaced, as mounting protests calling for Fedorov's reinstatement piled pressure on the Ukrainian president.

Fedorov was offered other posts in the government, including deputy prime minister for military innovation, but the 35-year-old has said he would only return as defence minister.

Hours before Fedorov posted his video, Zelensky confirmed he had asked parliament to confirm acting defence minister Yevhenii Khmara in the post. A vote was due to be held later on Wednesday.

Fedorov gave no indication as to his own political plans in the YouTube video, instead focusing on the need for a "legal, safe, and realistic mechanism that will allow Ukraine to restore a full democratic process even amidst a prolonged war".

His call for elections has angered some Ukrainian commentators, including those who backed Fedorov's return as defence minister. One of Zelensky's detractors argued that even the government's fiercest opponents understood that holding elections during the war was impossible.

Zelensky's five-year presidential term ended in May 2024, but under Ukraine's constitution no elections can be held while martial law is in place.

When US President Donald Trump suggested last year that Kyiv was "using war" to avoid a vote, Zelensky said Ukraine was "ready for elections" within 60-90 days if its allies helped guarantee security for them.

Holding a fair and safe vote would present considerable challenges for a country under repeated Russian bombardment and many Ukrainians saw Trump's remarks as reflecting Vladimir Putin's narrative that Zelensky is not a legitimate leader.

Mykhailo Fedorov argued that Ukraine was facing a "systemic crisis of governance" and was "scared of change", and pointed to corruption in Ukraine as one of the things hurting the country's war effort.

A former minister of digital transformation, Fedorov set up a volunteer "IT Army of Ukraine" to launch cyber-attacks against Russians in the early days after Putin launched a full-scale invasion of Ukraine.

Later, he led a successful fundraising campaign called the Army of Drones and brought in elements of "gamification" to the war, designing a system that awarded Ukrainian military units with credits for hitting Russian targets.

As defence minister, his focus on drones, high-tech warfare and procurement continued, and saw him ask SpaceX founder Elon Musk to stop Russia from using Starlink satellites for drone attacks.

The move has been credited with causing considerable disruption to Russia's frontline operations and advance.

In a Facebook post shortly after his dismissal, Fedorov listed his achievements and said he would "continue... to defeat the enemy through asymmetry, speed of innovation, and organisational strength".

Ukraine has stepped up long-range attacks across Russia in recent weeks, launching hundreds of drones to target oil refineries and the warehouses of its largest online retailer Wildberries.

Moscow suffered "one of the most massive drone attacks in recent memory" on Saturday night when about 600 drones were launched at it, according to the regional governor Andrey Vorobyov.

During the most recent Ukrainian strikes across Russia nine people were killed and 23 injured, including a child, local officials said.

Early on Tuesday in the latest Russian attacks on Ukraine 17 people were killed.

UK Prime Minister Andy Burnham has reaffirmed full support for Ukraine after Moscow warned of "consequences" following confirmation British-made drones were used in recent Ukrainian strikes in Russia.(BBC)