KYIV, Oct 1 - Ukrainian troops on Saturday took the key bastion in occupied eastern Ukraine, a stinging defeat for Russia that prompted a close ally of President Vladimir Putin to call for the possible use of low-grade nuclear weapons.

The capture came just a day after Putin proclaimed the annexation of four Ukrainian regions - including Donetsk, where Lyman is located - and placed them under Russia's nuclear umbrella, Reuters reported.

Kyrylo Tymoshenko, deputy head of President Volodymyr Zelenskiy's office, on Saturday posted a video of Ukrainian soldiers outside the town council building in the center of Lyman.

"Dear Ukrainians - today the armed forces of Ukraine ... liberated and took control of the settlement of Lyman, Donetsk region," one of the soldiers said. At the end of the video, a group of Ukrainian soldiers throw Russian flags down from the building's roof and raise a Ukrainian flag in their place.

Hours earlier Russia's defense ministry had announced it was pulling troops out of the area "in connection with the creation of a threat of encirclement".

Russia had used Lyman as a logistics and transport hub for its operations in the north of the Donetsk region. Its capture is Ukraine's biggest battlefield gain since a lightning counteroffensive in the northeastern Kharkiv region last month.

The recent successes have infuriated Putin allies such as Ramzan Kadyrov, the leader of Russia's southern Chechnya region, who said he felt compelled to speak out.

"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," Kadyrov wrote on Telegram before Zelenskiy spoke.

Putin said last week that he was not bluffing when he said he was prepared to defend Russia's "territorial integrity" with all available means, and on Friday made clear this extended to the new regions claimed by Moscow.

Washington says it would respond decisively to any use of nuclear weapons.

Serhii Cherevatyi, spokesperson for Ukraine's eastern forces, said before the capture that Russia had 5,000 to 5,500 troops at Lyman but the number encircled could be lower.

Neither side's battlefield assertions could be independently verified.

Kadyrov said Colonel-General Alexander Lapin, the commander overseeing Lyman, was a "mediocrity" who should be stripped of his medals and sent to the front.

Ukraine says taking Lyman will allow it to advance into the Luhansk region, whose full capture Moscow announced in early July after weeks of grinding advances.

"Lyman is important because it is the next step towards the liberation of the Ukrainian Donbas. It is an opportunity to go further to Kreminna and Sievierodonetsk, and it is psychologically very important," Cherevatyi said.

The Donbas has been a major focus for Russia since soon after the start of Moscow's invasion on Feb. 24 in what it called a "special military operation" to demilitarize its neighbor.

Putin proclaimed the Donbas regions of Donetsk and Luhansk and the southern regions of Kherson and Zaporizhzhia to be Russian land on Friday - a swathe of territory equal to about 18% of Ukraine's total surface land area. (rt/ez)