FEARSOME anti-Putin Russian rebels have rushed to the new northern front to help Ukraine fend off Moscow's onslaught.
It comes as President Zelensky blamed "the world" for Russian forces seizing over 100 miles in just a week - their fastest advance in a year.
On Thursday, the Freedom of Russia Legion's deputy commander Maksimilian Andronikov - who goes by his callsign "Caesar" - revealed they have joined Ukraine on its northeastern frontier.
The battle-hardened guerrilla group has been cutting its teeth fighting in some of Ukraine's bloodiest battles and launching raids into Russia - but will now help to shore up Kyiv's defences in Kharkiv.
Caesar claimed that Russia was becoming more innovative and harder to hold off.
"They've learned the lessons of the war, they're using rather intelligent tactics," he said.
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The partisan leader also claimed that the enemy's recent successes were down to Ukraine's lack of artillery after months of delays of Western weapon shipment.
"We feel the deficit. We understand well that if it didn't exist, the enemy wouldn't have these successes here or in the Donbas," he fumed.
Putin's troops kicked off a new ground offensive on May 10 in the Kharkiv region after massing more than half a million troops at the border.
Ukraine is desperately trying to hold the line against the blitz - said to be the first stage of Russia’s summer offensive - as Putin throws troops into meatgrinder assaults.
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The speed of Moscow's new assault has underscored the acute ammunition and manpower shortages crippling Ukraine that have paved the way for the Kremlin's army to storm through.
According to data from the Institute of the Study of War, Russia gained 99 square miles in Kharkiv and 13 square miles in other areas between May 9 and 15.
Winnie, another Russian fighting for the anti-Putin legion said: "It's an unbelievable meat grinder that they're still [sending] their people into."
"The situation is difficult, the intensity is very high, there is fighting almost every ten minutes," said the mortarman.
One particularly grim innovation Russia is using in Kharkiv is its expanding use of aerial bombs, which are dropped from planes and usually packed with hundreds of kilograms of explosives.
Russia has vast Soviet-era stocks of the relatively cheap bombs.
Over the past several months, Russia has been able to grind out battlefield gains by hammering frontline towns and infantry positions with aerial bombs.
"Today, four guided aerial bombs came in, about 500 metres away. I was on the ground, and it started vibrating, I was thrown upwards - and I'm not small," Winnie said.
Who are the Russian rebels?
DETERMINED to take the fight to Putin, Ukraine-supporting Russian partisans have been repeatedly launching raids and assaults in Russia's borderlands.
The "freedom fighters" - which are independent groups of Russian citizens - were mostly formed in 2022 in response to Russia's invasion of Ukraine.
Since then, they have been going from strength to strength with the explicit aim of finally ending the Kremlin regime.
The rebel group first exploded onto the world stage in May 2023, when they charged across the border in tanks and led one of the biggest assaults on Russian soil since Putin rose to power.
In a dramatic days-long battle, the Freedom of Russia Legion (FRL) and the Russian Volunteer Corps (RDK)“liberated” eight villages in the Belgorod region, even taking Russian soldiers captive and humiliating Vlad on his home turf.
On March 12, the anti-Putin paramilitaries returned with more men, more tanks and a new Siberian battalion by their side to lead a two-pronged assault into their homeland in the border regions of both Belgorod and Kursk.
They stated their goal was to disrupt Putin's sham elections and "take our land back from [Putin's] regime centimetre by centimetre".
The battle lasted for over a week and they seized key villages before later retreating but keeping the pressure on Russian defences with small-scale raids.
Their political head, Ilya Ponomarev, told The Sun at the time: "The military objective here is to test Russia's new defences but the primary objective here is political.
"To do it before the so-called 'elections' will inspire and mobilise Russians to our side to join our ranks.
"We will not support these sham elections as they try to legitimise Putin's power."
They are also fighting for Ukraine in some of its bloodiest battles raging across its 600-mile front.
Ponomarev, previously described the FRL to The Sun as a "well established military force" that was "by far the largest resistance network inside Russia".
Last December, The Sun spoke to Caesar who vowed that Russia is ready to explode and promised to light its fuse in 2024.
Caesar, 49, - who has become the face of their armed assaults into Russia - added: "That's why Putin is very, very afraid of us."
He warned Russia that his "warriors" will stop at nothing to “free our motherland” and “save Ukraine” in the process.
"We are the first military regiment who clearly and loudly says that only a struggle by force can change the power in Russia."
The commander said they have a combined force of thousands of brave Russians behind them - those who are willing to risk their lives to secure a “brighter future” for Russia.
Last night, the longest air raid siren of the war sounded above Ukraine's second-largest city of Kharkiv as it was pummelled with a missile and drone barrage.
Residents were urged to take cover during the longest air raid alert since the war began as Vlad's army pelted civilians with rockets.
Russian forces are also driving towards the towns of Lyptsi and Vovchansk north of Kharkiv - moves that Zelensky branded the "world's fault".
The embattled leader was visiting wounded troops in the region when he said the situation has become "very serious".
"It's the world's fault. They gave the opportunity for Putin to occupy. But now the world can help," he told as he pushed for more military support.
Vitaly Ganchev, an illegally Russian-appointed official in Ukraine, said that Russian troops were “already on the outskirts” of Lyptsi.
There are also disturbing reports from Ukraine that Russia has been abducting and killing civilians in its battle for the border town of Vovchansk.
Interior Minister Igor Klymenko via Telegram, “Russian troops are taking civilians prisoner.”
Klymenko added: “There are reports of the first shootings of civilians by the Russian military.”
It comes as The Sun spoke to a military experts who warned the Kharkiv offensive is a ploy to drag Kyiv's forces further north while he plots to seize entire of the east in just weeks.
Professor Michael Clarke, former director of the UK think-tank RUSI, believes that in coming weeks the Kremlin's troops will pound the Donbas from the north and south in a pincer movement to take it all.
General Sir Richard Sherriff, ex-deputy supreme allied commander of Nato, is also fearful that Putin's ultimate next goal is the east not the north.
He told The Sun: "The latest offensive looks to be causing real problems for the Ukrainians.
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"It could allow the Russians - if they launched an effective attack in the north, to force Ukraine to thin out its defences around the southeast [of Donbas].
"That could allow Russia to concentrate its forces in the Donbas and break through there and encircle Ukrainian defences while making significant ground."