Oleh, the commander of a Ukrainian mechanized infantry unit dug into trenches west of Kherson, is confident his Russian foes will be forced to abandon the strategic port by winter weather, logistical logjams and the threat of encirclement.

But neither he nor his men think the Russians will go quickly or quietly and nor do they intend to let them.

His comments raise the spectre of a bloody slog in the coming weeks for control of a key city on the west bank of the Dnipro River which acts as a gateway to the peninsula of Crimea annexed by Russia in 2014.

"They will keep fighting. They will defend their positions as long as they have the ability to do so," said Oleh, 26, a battle-hardened major who has risen through the ranks since enlisting as a teenager 10 years ago. "It will be a hard fight."

Kirill Stremousov, deputy head of the Russian-installed administration in Kherson region, said on Thursday that he hoped Russian forces would put up a fight.

"If we leave Kherson, it will be a huge blow," he added, in comments broadcast by Russia's RT television.

The contest for the only provincial capital seized by Moscow in the full-scale invasion launched on Feb. 24 may be one of the most consequential of the war so far.

For Russian President Vladimir Putin, it would be another setback following a series of significant battlefield losses since mid-August.

With control of the Dnipro's west bank, military experts said, Ukrainian forces would have a springboard from which to seize a bridgehead on the east side for an advance on Crimea.

Crimea is home to Russia's Black Sea fleet and Kyiv has made the peninsula's recovery its sworn goal.

Were Kherson to fall in the counter-offensive, the experts added, it would also be a political humiliation for Putin, as Kherson is one of four partially occupied regions of Ukraine that he announced would be part of Russia "forever" with great fanfare on Sept. 30.

"It would be a massive blow, primarily politically," said Philip Ingram, a retired senior British military intelligence officer. "And it would cost him (Putin) militarily. If the Ukrainians were able to get a bridgehead on the east side of the Dnipro, that would be even worse for the Russians."

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The Ukrainians "will be able to hammer the Russians defending the approaches to Crimea," said retired US General Ben Hodges, a former commander of US Army forces in Europe.

A US official, speaking on condition of anonymity, said it appeared the Russians already had begun "an organized, phased withdrawal" from the Dnipro's west bank.

ITCHING TO ATTACK

Thousands of civilians from the city and surrounding areas have been evacuated to the east side of the Dnipro in recent weeks after Russian-appointed occupation authorities warned of the dangers posed by Ukrainian advances.

On Friday, Putin publicly endorsed the evacuation that Kyiv says has included forced deportations of civilians out of Russian-occupied territory - a war crime - which Russia denies.

Occupation authorities also have relocated administrative offices and records to the east bank, and a Western source, speaking on condition of anonymity, said most Russian commanders had transferred their bases as well.

The US official and Ukrainian commanders said the Russians had been reinforcing their front lines, including deploying recently mobilized reservists, in a bid to better protect the withdrawal.

Some Ukrainian soldiers believe the poorly trained Russian reservists are being sent forward "like lambs to the slaughter", while more experienced troops are digging into defensive lines further back, according to the US official.

An orderly pullout could prove challenging for the Russians, requiring coordination, deception to conceal movements, communications discipline, and intense artillery barrages to suppress Ukrainian advances.

But Ukrainian troops could also face serious obstacles that could stall their takeover of Kherson, including booby traps and concentrated Russian artillery and rocket fire from the east bank, Hodges said.

As the sides on Friday fought intermittent artillery duels, Oleh's 100-man unit took advantage of unusually mild weather to clean weapons and install floorboards in earth-and-log-covered bunkers that are lined with thermal insulation and feature portable generators and wood-burning stoves.

The unit, with six armoured personnel carriers, took its positions in September after Ukrainian forces drove Russian troops back to Kherson's border with Mykolaiv province.

Oleh said the Russians were running short of time, as January would bring ice floes down the Dnipro that could block ferry operations.

He was impatient to strike the enemy's weak points to induce panic among reservists that could turn into a rout.

"If we don't start an attack, they will just keep sitting there," he said. "The mobilized ones are good for us because they generate panic. Panic is infectious like a disease. It spreads."