Ukraine's former defense chief Mykhailo Fedorov has called for wartime elections in what many see as the most direct political challenge to President Volodymyr Zelenskyy since Russia launched its full-scale invasion nearly four-and-a-half years ago.

Fedorov, who was unexpectedly sacked last month as part of a government reshuffle, said Tuesday that "democracy cannot be held hostage by Russia," according to a Reuters translation.

"We must find a legal, safe and realistic mechanism that will allow Ukraine to renew its full democratic process even in the conditions of a long war," Fedorov said.

His comments, which did not mention Zelenskyy by name, were delivered in a more than nine-minute video posted to YouTube. Ukrainian law currently prohibits holding elections during wartime.

A former ally of Zelenskyy, Fedorov is widely seen as a popular figure in Ukraine. His dismissal in late July after roughly six months in post prompted protests in major cities across the country.

An advocate of the country's drone warfare, the 35-year-old has been credited with energizing the defense ministry and spearheading a drive against corruption.

Fedorov's statement came just hours after Zelenskyy confirmed he had asked parliament to confirm Yevhen Khmara as Ukraine's new defense minister in office on Wednesday, effectively closing the door on a Fedorov comeback.

Fedorov said Ukraine faced a "systemic crisis of governance" and condemned official corruption for hurting the country's war effort, without providing the names of any lawmakers.

Zelenskyy has not yet responded to Fedorov's video statement. CNBC has contacted Ukraine's Foreign Ministry for comment and is awaiting a response.