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It could take Russia years to seize control of the four annexed regions of eastern mainland Ukraine it has laid claim to, and it would likely cost Moscow up to 1.8 million casualties to do so, the head of the British Army has said.

If they carry on as they are, it will probably take the Russians five years to grind their way to their minimum objectives of the four oblasts, General Sir Roly Walker, the new British Army chief, said during the Royal United Services Institute (RUSI) Land Warfare Conference on Tuesday.

Russia has annexed four regions, or oblasts, of eastern Ukraine, although it does not control all of Donetsk, Luhansk, Kherson and Zaporizhzhia. Moscow also controls Crimea, the peninsula to the south of mainland Ukraine which it annexed in 2014. Russia's control over the regions is not internationally recognized, and Kyiv has vowed to reclaim the annexed territories.

At the current rate of attrition of dead and wounded, that puts them probably well north of 1.5 million casualties to achieve that, with untold billions of lost equipment, Walker said. There has got to be more things for Russia to worry about than losing the best part of 1.5 to 1.8 million people for a slice of Ukraine.

Newsweek has reached out to the Russian Defense Ministry for comment via email.

Russia has been slowly but steadily advancing in eastern Ukraine at a huge cost to its military personnel. Although it is very difficult to pinpoint accurate casualty figures, Ukraine's military indicates Russia's highest casualty counts have come in recent months as its forces bear down on Ukraine's defenses in the east, while also launching a cross-border push into the northeastern Kharkiv region

Moscow is waging what is known as a war of attrition and experts say the Kremlin hopes to outlast Ukraine's defenses. Kyiv has a smaller pool of potential recruits to replenish its armed forces, and is heavily dependent on Western military aid in terms of equipment and ammunition.

Also speaking on Tuesday, Admiral Sir Tony Radakin, the chief of the U.K.'s defense staff, said Russia's military has now sustained an estimated 550,000 casualties in the nearly two and a half years of full-scale war in Ukraine.