In the deserted central square in Slovyansk in Ukraine’s eastern Donbas region, a few elderly residents are trying to find a way out of the city while they still can.

“Where are the evacuation buses?” asks Nadyezhda Ivanovna, 78, as artillery fire booms in the distance. “I want to leave now,” she said. “There will be a big war.” 

Vladimir Putin’s forces have begun an offensive to take the Donbas, having been repelled by Ukrainian forces around Kyiv and blocked in their push south-west towards the coastal city of Odesa.

Volodymyr Zelensky, Ukraine’s president, confirmed the start of the offensive late on Monday after a sharp rise in attacks on Ukrainian troops and infrastructure in the territory that for eight years has been at the centre of the bitter conflict with Russian-backed separatists.

“A very large part of the entire Russian army is now focused on this offensive,” he said in a video address to the nation. “No matter how many Russian soldiers are driven there, we will fight.”

Kyiv and foreign defence officials believe the campaign marks the start of a new phase of the almost two-month-old conflict, as Moscow seeks to capture territory in the two Donbas regions it does not yet control, including the battered port of Mariupol.

Many Ukrainians believe Putin, who invaded their country on the pretext of liberating it from “Nazis”, wants to notch up a victory in the Donbas before the May 9 holiday that marks the Soviet Union’s defeat of Nazi Germany. 

“At minimum, we understand that Putin wants to take the Donetsk and Luhansk regions,” said Tatiana Ihnatchenko, spokesperson for the military administration in the Donetsk region. “The maximum goal is to make a corridor to Crimea. And we understand that as long as Putin is alive, he wants all of Ukraine.” 

Before Zelensky’s announcement that the Donbas assault was under way, John Kirby, the Pentagon’s press secretary, said Moscow had been preparing the ground for the offensive, moving troops, heavy artillery, command and control enablers and air support into the region.

Ukraine’s former defence minister Andriy Zagorodnyuk, an adviser to the country’s government and chair of the Centre for Defence Strategies think-tank, said that after failing to take Kyiv, strategic cities such as Odesa and Mariupol, and “terrorising the population” with air strikes, Moscow was shifting to a fourth plan.

“Plan D is to collect forces in one location to capture that location,” he added. “It was a huge surprise to the world that the Russian army can’t fight properly, they want to show they can.

“If they amass troops of 80,000, which is what we are hearing, that’s a huge force.” 

That would be the largest concentration of Russian troops in one area since Moscow launched its full-blown invasion of Ukraine on February 24. However, Zagorodnyuk said the Russian forces were “bringing in old equipment, including some that goes back to the 1960s. To say that the outcome is already known is not the case.”

UK military intelligence said on Monday Ukrainian resistance had “tested Russian forces and diverted men and material”, and that Russian commanders would be “concerned by the time it’s taking to subdue Mariupol”.

Just as in the assault on Kyiv, analysts said Russia’s advance towards the Donbas had been thwarted by Ukrainian forces in the north-east around Izyum and Kharkiv, and in the south by about 2,000 fighters holding out in Mariupol. 

Ihnatchenko, who works in Kramatorsk, said residents in the Donbas had noticed the build-up of Russian armour. “With their eyes they can see in Donetsk how many tanks, Grads [rocket launchers] are coming,” she added. “They understand they are heading toward us.” 

People in Slovyansk, which sits on the Ukrainian side of the “line of contact” — the trenches and fortifications sliced through Donbas since 2014 — have been fleeing the city in recent weeks. 

Slovyansk was the first major Ukrainian city to fall to Russia when the separatists’ war began, and the first to be recaptured by Ukraine’s troops that July. 

When Kyiv regained control, officials found mass graves of civilians, and heard accounts of the sort of atrocities now being uncovered on a larger scale in Kyiv’s suburbs. 

In the 2014 conflict loyalties in the Donbas were divided, with some in the region that borders Russia feeling closer to Russia than Ukraine. But not any more, officials say.

“The majority of people here condemn the Russian forces, having seen their atrocities in Russian-speaking cities such as Kharkiv and Mariupol,” said Vadym Lyakh, Slovyansk’s mayor. He estimated that at least 60 per cent of residents had already left. 

In Kramatorsk, the Donetsk region’s biggest Ukrainian-controlled city, a Russian missile attack on the railway station this month killed 58, including children, and injured 114 as they tried to leave.

Oleksandr Goncharenko, the city’s mayor, estimated only about a quarter of the town’s 157,000 people remained. “We have evacuated most of the population to prevent a humanitarian catastrophe as happened in Mariupol,” he said, speaking from a basement bunker. 

Some servicemen protecting the region said they were confident of their ability to repel Russian troops. 

“Our formation in Donbas is the most capable part of our forces,” said Oleksandr V Danylyuk, a security and defence adviser to Ukraine’s government, and veteran of the 2014 battle to recapture Slovyansk. 

“They know the terrain and are used to fighting in this operational environment,” he added. “It is highly unlikely that the Russians can achieve any significant victories before May 9, their national Victory Day before which they so badly want to score some kind of victory in Ukraine.”

Additional reporting by Felicia Schwartz in New York