President Vladimir Putin on Sunday nominated economist Andrei Belousov to replace Sergei Shoigu as Moscow's defence chief, a surprise move more than two years into a conflict that Russia hoped would be wrapped up in a matter of days.

"The key task, of course, remains achieving victory and ensuring that the military-political goals of the special military operation, set by the president, are achieved. In this respect, I want to especially emphasise: with minimal human losses," Belousov told a session of Russian lawmakers on Tuesday.

Moscow rarely talks about the losses it has sustained through the conflict, which it still calls a "special military operation".

It last gave an official figure in September 2022, when it said 5,937 soldiers had been killed in combat.

But several independent analyses and assessments by Western intelligence services put Russian deaths well into the tens of thousands.

The BBC and the independent Mediazona outlet say they have verified the deaths of at least 50,000 Russian soldiers through tracking official local reports and announcements of soldiers' deaths in local newspapers, by regional officials and on social media pages.

Ukraine's General Staff on Monday said it had inflicted a record number of losses on Russian forces over the last 24 hours after Moscow launched a major new ground assault on the northeastern Kharkiv region.

Ukrainian President Volodymyr Zelensky said in February that around 31,000 Ukrainian soldiers had been killed in the first two years of the conflict.

AFP is unable to verify those figures.

Following confirmation hearings in Russia's rubber-stamp parliament, Putin signed a decree on Tuesday confirming Belousov as the country's new defence chief.

Belousov also said that Moscow was not discussing a new wave of mobilisation, "or some kind of emergency measures" to boost troop numbers.

Russia has a manpower advantage over Ukraine on the battlefield.

The Kremlin has indicated Belousov, an economist with no military background, will not be involved in making operational decisions over the campaign, which will be the remit of the army's General Staff.

- From security to shipbuilding -

Separately on Tuesday, Putin appointed his powerful ally Nikolai Patrushev as his aide responsible for shipbuilding -- an apparent demotion from his previous role as Security Council secretary.

Patrushev, 72, is a shadowy but influential figure. Like Putin, he has a background in the intelligence agencies and is one of the regime's most strident anti-West hawks.

Kremlin spokesman Dmitry Peskov acknowledged that Patrushev's new role in charge of shipbuilding was work "of a different nature".

"But this is an absolutely strategic sector -- huge, very complex," he added during a briefing with reporters.

Although an apparent step down, the role brings Patrushev into the presidential administration and ensures he will maintain direct contact with Putin.

Outgoing defence chief Shoigu will replace him at the Security Council.