Zelenskiy's major reshuffle of his government on Thursday, which included the appointment of a new prime minister, unsettled the country's military leadership and trigged a public outcry.
It was an unwelcome difficulty after Ukraine has gained traction in its fight against Russia's more than four-year-old invasion.
The surprise departure from the defence ministry of Mykhailo Fedorov, a youthful and popular member of the government, prompted thousands of people to demonstrate against his dismissal in cities across Ukraine on Thursday.
Further street protests were expected on Friday.
Fedorov, 35, who was in the post for just six months, is widely seen as the driving force behind Ukraine's swift and successful technological innovation and other measures, such as fighting military corruption, that have brought fresh hope in the war for Ukrainians.
Relations between Fedorov and General Oleksandr Syrskyi, the commander of Ukraine's armed forces who started his military career in the former Soviet Union, had broken down, according to Zelenskiy, and made Fedorov's position untenable.
Zelenskiy said he had asked Major General Yevhen Khmara, acting head of the state's security service and a highly regarded special operations expert, to take over the defence minister's duties.
Zelenskiy said late on Thursday he would ask parliament to formally approve Khmara's appointment as defence minister, as required by law.
That step could be held up by bureaucratic hurdles, however.
Ukrainian law requires the defence minister to be a civilian, so a serving soldier or security service officer must leave active service before being formally appointed.
Also, MPs will be on summer recess through mid-August.
Khmara has been in charge of the SBU security service since January.
He had previously led the SBU's elite Alpha special forces unit and is known for being an architect of Operation Spiderweb, one of Ukraine's most spectacular attacks when it struck Russian air bases in 2025.
He joined the Alpha unit in 2011 and became its commander in 2023 before being promoted to major general the following year.
Moscow's response to its battlefield difficulties and Ukraine's targeting of Russian oil facilities, which has caused severe fuel shortages, has focused in part on relentless strategic bombing of civilian areas of Ukraine.
Two people were killed and 10 others injured, including children, in an overnight Russian missile attack on Ukraine's southern port city of Odesa, regional military administration head Oleh Kiper said.
In the Zaporizhzhia region, two people were killed and five more were injured in a strike, according to Zelenskiy.
He said three people were injured as a result of Russian shelling in the northeastern Kharkiv region.
Officials said more people were injured in Russian strikes on five other regions of Ukraine.
Meanwhile, Russia's defence ministry said that air defences downed 243 Ukrainian drones overnight into Friday.
Three civilians were killed and seven others injured in Ukrainian drone attacks over the previous 24 hours, according to Vladimir Saldo, the Moscow-appointed head of the Russia-occupied part of Ukraine's Kherson region.
with Reuters
