Russia is set to annex four more regions of Ukraine within hours, sparking fears of a "dangerous escalation" in the war – but experts insist the invaders are about to face a massive defeat.

President Vladimir Putin will hold a signing ceremony later today which will see Russia formally annex the regions of Luhansk and Donetsk in Ukraine's east, and Zaporizhzhia and Kherson in the south.

The move comes after rushed referendums were held in the regions last week, with the overwhelming majority of votes supporting annexation.

However, the referendums were universally condemned as shams, with German Foreign Minister Annalena Baerbock claiming that some residents had been taken to vote at gunpoint, which she described as "the opposite of free and fair elections".

Meanwhile, UN Secretary-General Antonio Guterres recently warned that the annexation of the regions would mark a "dangerous escalation" that would undermine the possibility of peace, given Russia has declared that any future attacks on the regions would be considered an attack on Russia itself, and could even be defended with nuclear weapons.

"Putin has given us a choice – accept the redrawing of border through force and avoid nuclear threats (for now) or reject sham referendums and help Ukraine preserve itself and nation-state concept and accept nuclear risks," former US arms control official Jon Wolfsthal said in a tweet regarding the crisis.

But while the situation seems to be deteriorating, a string of experts have claimed that Russia is in fact facing a new, major defeat which could turn the tables on the brutal invasion.

'Imminent, painful defeat'

While Russia prepares for annexation, Ukrainian troops have been ramping up plans to recapture the key town of Lyman in the north of the Donetsk region.

If successful, it would pose a major setback for Putin's plans for the Donbas region, which became a major focus of the invasion after Russia's early failures.

Initially, Russia planned to seize the capital Kyiv and second city Kharkiv, but troops were met with fierce resistance, causing Russia to quickly pivot its strategy and instead zero in on the industrial Donbas region – including Donetsk and Luhansk – an area where Putin falsely accused Ukraine of committing genocide.

• NZ joins 'sham' condemnation over Russia referenda
• Ukraine accuses Russia of carrying out "terrorist attack" on two major gas pipelines

But if Ukraine succeeds in capturing Lyman, it would provide a major moral boost – and could also lay the foundation for Ukraine to progress into the nearby Luhansk region, which would leave Putin's plans for the area in tatters.

And it seems like things could be going Ukraine's way, with former Swedish prime minister Carl Bildt declaring on Twitter that: "The situation looks increasingly precarious for Russian forces in Lyman as Ukrainian forces are about to cut them off", adding that: "Another painful defeat for the Russian invasion forces is looming".

That sentiment was echoed by US think tank the Institute for the Study of War, which agreed Russia faced "imminent defeat" in Lyman.

The body claims Ukraine has already taken villages nearby Lyman, and noted that Russian military bloggers had expressed "increased concern" over the Lyman situation this week, "suggesting that Russian forces in this area may face imminent defeat".

"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," the institute said.

Russian recruits take a bus near a military recruitment centre in Krasnodar, Russia. Photo / AP

Relatives say goodbye to Russian citizens drafted during the partial mobilisation as they depart to their military units. Photo / Getty Images

"Additional Russian losses would further erode morale amid the country's mobilisation."

It added that Russia was relying on "newly mobilised and undertrained" recruits to bolster "severely degraded remnants of various units" which were previously thought to be comprised of Russia's top fighters.

And it's not just Western sources talking about Lyman, with the Moscow-installed head of the city's administration admitting yesterday that it was increasingly "difficult" for Russia to hold the territory in the face of Ukrainian advances.

New threat facing Putin

While Ukrainian forces are threatening Putin's campaign, the leader is also facing an unprecedented threat from within.

Russian recruits gather inside a military recruitment centre of Bataysk, Rostov-on-Don region, south of Russia. Photo / AP

According to a damning new survey by Russia's independent Levada Centre, the majority of Russians do not support the president's decision to force 300,000 military reservists into battle, with more and more citizens convinced the war is not going their way.

The mobilisation of the reservists – including retirees – caused feelings of fear, alarm or shock in 70 per cent of those polled, with 66 per cent believing Putin could eventually expand the mobilisation to the entire nation.

Muscovites await their departure to the military training centre at the mobilisation centre in Moscow. Photo / Getty Images

Meanwhile, 31 per cent believe the invasion is failing, with almost half supporting peace talks, and a separate Levada poll also revealed Putin's approval rating had plummeted as a result of the mobilisation.

Major announcement within hours

A signing ceremony to incorporate Luhansk, Donetsk, Zaporizhzhia and Kherson into Russia will be held in the St George Hall of the Grand Kremlin Palace at 3pm Russian time (1am NZ time).

"Separate agreements will be signed with the two Russian-backed separatist leaders from the east and the two Russian-appointed officials from the south," Kremlin spokesman Dmitry Peskov said in a statement.

He added that the ceremony would be followed by a "voluminous speech" by Putin himself, which will be keenly watched by experts in a bid to find clues regarding his planned next moves as the invasion rages on.

- Alexis Carey, news.com.au