KYIV  — Ukrainian troops said they had taken the key bastion of Lyman in occupied eastern Ukraine, a stinging defeat that prompted a close ally of Russian President Vladimir Putin to call for the possible use of low-grade nuclear weapons.

The capture on Saturday came just a day after Mr. Putin proclaimed the annexation of nearly a fifth of Ukraine — including Donetsk, where Lyman is located — and placed the regions under Russia’s nuclear umbrella. Kyiv and the West condemned the ornate ceremony as an illegitimate farce.

Ukrainian soldiers announced the capture in a video recorded outside the town council building in the center of Lyman and posted on social media by Kyrylo Tymoshenko, deputy head of President Volodymyr Zelensky’s office.

“Dear Ukrainians — today the armed forces of Ukraine … liberated and took control of the settlement of Lyman, Donetsk region,” one of the soldiers says. At the end of the video, a group of 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 the lightning counteroffensive in the northeastern Kharkiv region last month.

Seeking to capitalize on Ukraine’s recent gains, Zelensky promised more quick successes in the Donbas, which covers the Donetsk and Luhansk regions that are largely under Russian control.

“Over the past week, the number of Ukrainian flags in Donbas has increased. There will be even more a week’s time,” the president said in an evening video address.

US Defense Secretary Lloyd Austin cheered Lyman’s capture, saying it would create new problems for Russia’s military. “We’re very encouraged by what we’re seeing right now,” Mr. Austin told a news conference on Saturday.

Mr. Austin noted that Lyman was positioned across supply lines that Russia has used to push its troops and materiel down to the south and to the west, as the Kremlin presses its more than seven-month-long invasion of Ukraine.

“Without those routes, it will be more difficult. So, it presents a sort of a dilemma for the Russians going forward.”

Mr. Austin did not say whether he thought Ukraine’s capture of Lyman might prompt Russian escalation, although US officials have widely denounced Russia’s nuclear rhetoric in recent days and President Joseph R.  Biden has publicly urged Mr. Putin not to use nuclear weapons.

Ukraine’s successes have infuriated Putin allies such as Ramzan Kadyrov, the leader of Russia’s southern Chechnya region.

“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,” Mr. Kadyrov wrote on Telegram before Mr. Zelensky spoke.

Other top officials, including former President Dmitry Medvedev, have suggested Russia may need to resort to nuclear weapons, but Mr. Kadyrov’s call was the most urgent and explicit.

Mr. 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.

Reuters could not verify either side’s battlefield assertions.

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,” Cherevatyi said. “It is an opportunity to go further to Kreminna and Sievierodonetsk, and it is psychologically very important.”

The Donbas has been a major focus for Russia since soon it launched the invasion on Feb. 24 that Mr. Putin calls a “special military operation” to demilitarize and “denazify” its smaller neighbor. The areas Mr. Putin claimed as Russian — the Donbas regions of Donetsk and Luhansk and the southern regions of Kherson and Zaporizhzhia — form a swath of territory equal to about 18% of Ukraine’s total surface land area. Kyiv vowed to continue liberating its land of Russian forces.

Retired US Army General Ben Hodges said a Russian defeat in Lyman represented a major political and military embarrassment for Mr. Putin. — Reuters

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