By Yuliia Dysa and Max Hunder

KYIV, Jan 5 (Reuters) - President Volodymyr Zelenskiy replaced the head of his powerful security agency on Monday, pressing ahead with a reshuffle of top officials as Russia's war in Ukraine nears the four-year mark with no end in sight despite intense diplomacy.

The removal of Vasyl Maliuk, who oversaw a series of assassinations and a surprise operation codenamed "Spiderweb" that destroyed Russian warplanes, shakes up the leadership of a vast security agency at the heart of Ukraine's shadow war.

Zelenskiy named Major-General Yevhenii Khmara as the acting head of the Security Service of Ukraine (SBU), which Maliuk had run since 2022. The agency described Khmara as an experienced commander who had served in the elite Alfa combat unit since 2011 and led it since 2023.

SOME COMMANDERS STAND BY MALIUK

Zelenskiy said on X he had asked Maliuk to focus on combat operations, adding: "There must be more Ukrainian asymmetric operations against the occupier and the Russian state, and more solid results in eliminating the enemy."

Zelenskiy last week named military spy chief Kyrylo Budanov as his chief of staff to replace Andriy Yermak, who was ousted in November amid a corruption scandal but denied wrongdoing.

New defence and energy ministers will also be named in a reshuffle that had been put on hold during negotiations on a possible peace deal that produced no sign of a breakthrough.

Russia, which invaded Ukraine in February 2022, holds about one-fifth of Ukrainian territory including Crimea, which it seized in 2014. Kyiv says the advances have come at huge losses for Russia.

Zelenskiy has also appointed First Deputy Foreign Minister Serhiy Kyslytsia as a first deputy to Budanov, bringing a member of Ukraine's negotiating team into his administration.

Some commanders rushed to Maliuk's defence after reports of his looming exit circulated at the weekend.

Drone forces chief Robert Brovdi said replacing him risked weakening "one of Ukraine's key contemporary heavyweight" agencies fighting Russia.

Solomiia Bobrovska, a lawmaker on the defence, security and intelligence committee, said she did not think Maliuk's removal was "necessary today".

"In my opinion, under Maliuk's leadership, the SBU has been doing its job successfully in the current political situation, balancing between political tasks, combat operations and economic issues," she told Reuters.

She said Zelenskiy had effectively bypassed parliament by appointing Khmara as placeholder by decree.

MALIUK OVERHAULED SBU

Maliuk, a career SBU official, overhauled the SBU after replacing Ivan Bakanov, who was dismissed by Zelenskiy in July 2022 for failing to root out Russian spies.

Maliuk's most sophisticated operation, Spiderweb, used small drones hidden in cargo trucks to attack dozens of Russian highly valuable strategic bombers at airbases far from Ukraine's usual strike capabilities in May 2025.

A senior Ukrainian official, speaking anonymously, told Reuters Maliuk had been asked to run the smaller foreign intelligence service instead of the SBU, but declined.

The foreign spy service has no chief after its previous head was put in charge of the military intelligence agency. The head of the border guard service has also been replaced, another sign of the extensive security and defence reshuffle underway.

Mykhailo Fedorov, Zelenskiy's choice as defence minister, has not yet been approved by parliament. Zelenskiy said on Monday he wanted him to focus on technology and innovation to "counter Russia's desire to make this war endless."

(Reporting by Yuliia Dysa in Warsaw, Max Hunder and Tom Balmforth in London and Dan Peleshchuk in Kyiv; Editing by Gareth Jones, Toby Chopra and Timothy Heritage)