(Adds more Kadyrov quotes, context)
By Felix Light
LONDON, Oct 1 (Reuters) - Ramzan Kadyrov, head of Russia's region of Chechnya, said on Saturday that Moscow should consider using a low-yield nuclear weapon in Ukraine after a major new defeat on the battlefield.
As Russia confirmed the loss of its stronghold of Lyman in eastern Ukraine, Kadyrov slammed top commanders for their failings and wrote on Telegram: "In my personal opinion, more drastic measures should be taken, right up to the declaration of martial law in the border areas and the use of low-yield nuclear weapons".
He was speaking a day after President Vladimir Putin proclaimed the annexation of four Ukrainian regions - including Donetsk, where Lyman is located - and placed them under Russia's nuclear umbrella, saying Moscow would defend the lands it had seized "with all our strength and all our means".
Russia has the world's largest atomic arsenal, including low-yield tactical nuclear weapons that are designed to be deployed against opposing armies.
Other top Putin allies, including former president Dmitry Medvedev, have suggested that Russia may need to resort to nuclear weapons, but Kadyrov's call was the most urgent and explicit.
The influential ruler of the Caucasus region of Chechnya has been a vocal champion of the war in Ukraine, with Chechen forces forming part of the vanguard of the Russian army there. Kadyrov is widely believed to be personally close to Putin, who appointed him to govern restive Chechnya in 2007.
In his message, Kadyrov described Colonel-General Alexander Lapin, commander of the Russian forces fighting at Lyman, as a "mediocrity", and suggested that he should be demoted to private and stripped of his medals.
"Due to a lack of elementary military logistics, today we have abandoned several settlements and a large piece of territory," he said.
Kadyrov said that two weeks before he had raised the possibility of a defeat at Lyman with Valery Gerasimov, chief of Russia's general staff, but that Gerasimov had dismissed the idea.
Russia's Defence Ministry on Saturday announced a withdrawal from Lyman, a major stronghold and logistical hub for Russian forces in Ukraine's Donetsk region saying that a Ukrainian advance had threatened its units with encirclement.
It was the latest in a series of battlefield humiliations for Russia, after its forces were routed from Kharkiv region by a lightning Ukrainian counteroffensive last month.
After Russia's defeat in Kharkiv, Kadyrov said he would be "forced to go to the country's leadership to explain to them the situation on the ground" unless urgent changes were made in the conduct of the war.
Putin said last week he was not bluffing when he said he was prepared to defend Russia's "territorial integrity" with all available means. Washington says it would respond decisively to any use of nuclear weapons and has spelled out to Moscow the "catastrophic consequences" it would face. (Editing by Mark Trevelyan)