Meanwhile, Ukrainian authorities are accusing Russian troops of using flamethrowers against civilian targets in the Donbas.
Russian forces are increasingly relying on heavy Soviet-era weapons that can cause mass casualties and significant collateral damage in their bid to make headway in capturing eastern Ukraine.
Fierce, prolonged fighting is depleting resources on both sides, according to British and US defence officials.
Russian bombers are thought to have been launching hefty 1960s-era anti-ship missiles in Ukraine, the UK Defence Ministry said on Saturday.
The Kh-22 missiles, with a reported range of 1,000 kilometres, were primarily designed to destroy aircraft carriers and can carry a nuclear warhead.
When used in ground attacks with conventional warheads, they "are highly inaccurate and therefore can cause severe collateral damage and casualties," the ministry said.
It is unclear how many Kh-22s Russia has in its arsenal. By 2006, Ukraine has scrapped 423 missiles it inherited from the Soviet Union after deciding to decommission the weapon.
'Imperial appetites matter more'
Both sides have expended large amounts of weaponry in what has become a grinding war of attrition for the eastern region of coal mines and factories known as the Donbas, placing huge strains on their resources and stockpiles.
Russia is likely using the 5.5-tonne anti-ship missiles because it is running short of more precise modern missiles, the UK ministry said. It gave no details of where exactly such missiles are thought to have been deployed.
As Russia also sought to consolidate its hold over territory seized so far in the 108-day war, the US defence secretary said Moscow's invasion of Ukraine "is what happens when oppressors trample the rules that protect us all".
"It's what happens when big powers decide that their imperial appetites matter more than the rights of their peaceful neighbors," Lloyd Austin said during a visit to Asia.
"And it's a preview of a possible world of chaos and turmoil that none of us would want to live in."
Russian troops alleged to be using flamethrowers against civilians
Meanwhile, Luhansk's Ukrainian governor accused Russia of using incendiary weapons in a village in the eastern province, southwest of the fiercely contested cities of Sievierodonetsk and Lysychansk.
While the use of flamethrowers on the battlefield is legal, Governor Serhiy Haidai alleged the overnight attacks in Vrubivka caused widespread damage to civilian facilities and an unknown number of victims.
"At night, the enemy used a flamethrower rocket system — many houses burnt down," Haidai wrote on Telegram on Saturday. His claim could not be immediately verified.
Sievierodonetsk and neighbouring Lysychansk are the last major areas of Luhansk remaining under Ukrainian control. Haidai said Russian forces destroyed railway depots, a brick factory and a glass factory.
The Ukrainian army said Saturday that Kremlin forces were to launch an offensive on the city of Sloviansk in Donetsk province, which together with Luhansk makes up the area of the Donbas.
Moscow-backed separatists have forcefully taken control over the self-proclaimed republics of LNR and DNR in 2014.
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