President Volodymyr Zelensky has reshuffled the leadership of Ukraine's Security Service (SBU), replacing influential Vasyl Malyuk and nominating Maj-Gen Yevhenii Khmara as acting head.

The SBU is primarily concerned with internal security and counter-intelligence and, since the beginning of Russia's full-scale invasion in 2022, it has also played a prominent role in assassinations and sabotage attacks deep inside Russia.

Malyuk, who had run the SBU since 2022, gained a reputation for overseeing successful operations against Russia and for purging the SBU from alleged Russian double agents.

The latest reshuffles are the latest in a long series made by Zelensky since the start of the war.

Malyuk's name is known across Ukraine. After all, last June he co-ordinated the daring "Spider Web" attack, which saw more than 100 Ukrainian drones struck air bases deep inside Russian territory. Khmara, his replacement, was also involved in the preparation of that operation.

So it did not come as a surprise that Malyuk's dismissal earlier this week was widely criticised within Ukraine, including by several high-profile military commanders. The impression that the decision made by President Zelensky was short-sighted was cemented by reports that Malyuk had initially refused to step down voluntarily.

Although he has been dismissed from the position of SBU chief, he has been asked to continue leading special operations against Russia. Ukraine needed more "asymmetric operations against Russian forces", Zelensky said, adding that Malyuk was "best at this".

Khmara's appointment has gone some way in quelling fears about the SBU's future effectiveness. But former SBU operative Ivan Stupak argues his reputation - though solid - is limited to a certain sphere and that it is unclear whether he will be able to translate his experience to running the wider SBU.

Stupak also believes that Malyuk's dismissal is connected with the appointment of former spy chief Kyrylo Budanov as Zelensky's new chief of staff.

"Malyuk and Budanov are at loggerheads," he told the BBC, implying that Budanov had heavily influenced Zelensky's decision and that Khmara would now be "in the hands" of Budanov.

Other analysts disagree. Oleksandr Notevskyi, political analyst for Ukrainian media outlet Grunt, praised Khmara's appointment, pointing out his successes in heading Alpha, one of the top elite divisions of the SBU: "He's young, he's experienced, he is one of the authors of the Spider Web operation."

In a statement, the SBU described Khmara as a decorated and "experienced special-purpose officer" who was involved in liberating the Kyiv region in 2022 and fighting the Russians in the occupied region of Donetsk.

Khmara's appointment will need to be confirmed by the Ukrainian parliament, which may yet reject it. Other options are on the table, Notevskyi said.

After nearly four years of war, Ukraine is struggling to push back Russian forces in the conventional way - on the battlefield and along the long front line in the east of the country. It has had more success targeting oil fields, weapons factories and - as in the case of the Spider Web attack - air bases and valuable aircraft.

At this particular juncture of the war with Moscow, Notevskyi argued, "whoever leads the security service should be someone who is specialised in eliminating Russia's military capacities on the territory of Russia".

Last week, Zelensky announced significant changes to his top team. Mykhailo Fedorov was nominated as new defence minister, while foreign intelligence chief Oleh Ivashchenko replaced Budanov.

Many of Zelensky's reshuffles have been criticised by commentators as damaging to the country at a time of crisis.

Using a football analogy, Stupak argued that Zelensky was like a manager going to the substitute bench for replacements. "But his bench is quite short, and he's running out of people to use again and again," he said.

Additional reporting by Volodymyr Lozhko and Anastasiia Levchenko