(Bloomberg) -- Russian President Vladimir Putin may not have long to wait for the first major test of his decision to annex a swathe of Ukraine, with Kyiv’s forces threatening to encircle a pocket of the Donbas region he set out to conquer.

The Kremlin said Putin will on Friday sign what it called treaties with the Moscow-anointed leaders of four partially occupied provinces in Ukraine’s east. Ratification of their absorption into Russia would soon follow.

The move would follow this month’s illegal and hastily conducted referenda in those territories on whether to join Russia. Yet even as Putin prepares for a significant escalation of the conflict, Ukrainian, Western and Russian military analysts say Russian units are at risk of being enveloped in the Donetsk town of Lyman.  

The capture of a road and rail junction taken by Russian forces in late May would be a further set back of Putin’s publicly stated minimum goals for his Feb. 24 invasion -- namely to seize all of the Donetsk and Luhansk provinces, often referred to collectively as the Donbas region.

It would also open a path for further Ukrainian advances and force Putin to decide whether his Sept. 21 threat to use all means, including nuclear weapons, to protect Russia will apply to the newly appropriated territories in Ukraine.

“The offensive of the Armed Forces of Ukraine from the southern and western flanks carries the threat of blocking the only remaining supply route for the allied forces along the Svatovo-Makeevka-Terny-Liman road,” the Russian military blogger Rybar said in a Wednesday post. 

A person close to the Russian defense ministry said on Thursday that Lyman was likely to be lost within days.

Putin’s Sept. 21 speech left ambiguous whether his nuclear threat would extend to attacks on newly annexed territories. Russian officials have since redefined his pledge to defend the country’s integrity as a reference to the Russian state, rather than any particular border. 

At the same time, Putin announced a politically toxic mobilization to raise 300,000 new troops to send to the war, and Russia has become a prime suspect in damage to undersea Nord Stream natural gas pipelines that NATO allies called “reckless and irresponsible acts of sabotage.”

“Risks are always there,” when you live next door to a nuclear power, Ukraine’s Defense Minister Oleksii Reznikov told Ukraine’s Interfax news agency, but he added that after months of war it was no longer possible to intimidate Ukrainians. “I have been always telling our western partners - stop being afraid of Russians.”

On Wednesday, meanwhile, the US said it was sending a further $1.1 billion worth of arms to Ukraine, including ammunition and anti-drone weapons.

Ukraine’s President Volodymyr Zelenskiy called a meeting of his security council for Friday, likely to consider a response to the attempted annexations. 

Lyman is of military significance because it has acted as a launch point for Russian assaults on Slovyansk, one of the last Donbas population centers still under Ukrainian control, following the loss of Donetsk and Luhansk in 2014 and of Mariupol earlier this year.

Once again in Ukrainian hands it could provide a base for attacks toward the twin cities of Sievierodonetsk and Lysychansk about 60 km (37 miles) east, seized by Russia earlier this year in fierce attritional battles. The pressure is on for Ukrainian forces to make more territorial gains, before winter rains and a wave of new Russian conscripts makes that harder. 

“The collapse of the Lyman pocket will likely be highly consequential to the Russian grouping in northern Donetsk and western Luhansk oblasts and may allow Ukrainian troops to threaten Russian positions along the western Luhansk Oblast border and in the Severodonetsk-Lysychansk area,” said the Institute for the Study of War, a Washington think tank that monitors the war daily.

Ukrainian and Western military analysts saw the fall of Lyman as just a matter of time on Thursday, based on unverified reports that the last remaining road for Russian troops to escape from the pocket had come under Ukrainian artillery fire.

(Adds observation that Lyman likely to fall within days)

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