SEVERODONETSK, Ukraine (AFP) - Desperate evacuation attempts from eastern Ukraine were under way on Thursday (April 7) as authorities warned of an imminent Russian offensive, following the devastation around Kyiv that has shocked the world.

Russian troops have been withdrawing from around the capital and Ukraine's north, leaving a trail of destruction behind them, as they prepare for an expected assault on the country's southeast.

The scenes of carnage left behind by retreating troops in towns like Bucha have caused outrage and led to a wave of fresh sanctions against Moscow.

But on Thursday, Ukrainian President Volodymyr Zelensky warned that Moscow was undeterred and continued "to accumulate fighting force to realise their ill ambitions in (eastern) Donbass".

"They are preparing to resume an active offensive," he said.

Ukraine's deputy chief of staff of ground forces said on Thursday that Russia is likely to renew its attack on Kyiv if its forces succeed in taking full control of the eastern regions of Donetsk and Luhansk.

Speaking at an online briefing about efforts to defend the capital, Oleksandr Gruzevich said: “It is likely the enemy has not given up the goal of a second attack on Kyiv – there is such a threat.”

In the country's east, a Ukrainian official warned residents remaining there that time was running out to flee, saying that all villages in the region were under attack.

"These few days may be the last chance to leave," Luhansk regional governor Sergiy Gaiday wrote to residents on Facebook, cautioning that Russian forces were "trying to cut off all possible ways of taking people out".

Meanwhile the prospect of a negotiated end to the war seemed to fade as Russia accused Kyiv of changing its demands since face-to-face talks in Istanbul last month.

Foreign Minister Sergei Lavrov said a new draft agreement from the Ukrainian side suggested they were not interested in peace.

Ukrainian presidential adviser Mykhaylo Podolyak hit back on Twitter, saying that if Moscow wanted to show its readiness to talk, “it should lower the degree of hostility”.

Foreign Minister Mevlut Cavusoglu of Turkey, which hosted the discussions, said the images of bodies from Bucha and other areas had “overshadowed” what had been an “emerging positive atmosphere” in talks.

'Nowhere to go'

Gaiday said on Facebook that more than 1,200 people had been evacuated from Luhansk on Wednesday, but that efforts were being hampered by artillery fire, with some areas already inaccessible.

For those that unable to leave, he said, tonnes of food, medicine and hygiene products were being delivered as part of a massive humanitarian effort.

The head of the Donetsk Regional Military Administration said strikes had targeted aid points.

"The enemy aimed directly there with a goal to destroy the civilians," Pavlo Kyrylenko wrote on Facebook.

He added that people were heeding calls to flee and he would be coordinating evacuation to make it "faster and more effective". Shells and rockets were also slamming into the industrial city of Severodonetsk, the easternmost city held by Ukrainian forces.

"We have nowhere to go, it's been like this for days," 38-year-old Volodymyr told AFP standing opposite a burning building in the city.

More than 11 million people have been displaced since Russia invaded on Feb 24, aiming to seize the capital.

With that goal thwarted, it is instead trying to create a land link between occupied Crimea and Moscow-backed separatist statelets in Donbass.

Ukrainian forces are also regrouping for the offensive, including on a two-lane highway through the rolling eastern plains connecting Kharkiv and Donetsk.

Trench positions were being dug, and the road was littered with anti-tank obstacles.

"We're waiting for them!" said a lieutenant tasked with reinforcing the positions, giving a thumbs up.

Western allies have already sent funds and weapons to help Ukraine.

But Foreign Minister Dmytro Kuleba on Thursday made a fresh appeal for heavy weaponry, including air defence systems, artillery, armoured vehicles and jets.

“My agenda is very simple. It has only three items on it. It’s weapons, weapons, and weapons,” he told journalists ahead of a meeting with Nato ministers in Brussels.

'Brutality and inhumanity'

The evacuation calls are being fuelled by fears of fresh atrocities, after chilling discoveries in areas from which Moscow's troops have withdrawn.

US President Joe Biden said "major war crimes" were being committed in Ukraine, where images have emerged in recent days of bodies with their hands bound or in shallow graves.

"Civilians executed in cold blood, bodies dumped into mass graves, the sense of brutality and inhumanity left for all the world to see, unapologetically," Biden said.

In one of the worst affected towns, Bucha, some residents were still trying to learn the fate of loved ones, while others were hoping to forget.

Tetiana Ustymenko's son and his two friends were gunned down in the street, and she buried them in the garden of the family home.

"How can I live now?" she said.

The Kremlin denies responsibility for any civilian deaths and Putin on Wednesday accused Ukrainian authorities of "crude and cynical provocations" in Bucha.

But the German government pointed to satellite pictures taken while the town was still under Moscow's control, which appear to show bodies in the streets.

Russia's denials "are in our view not tenable", said German government spokesman Steffen Hebestreit.

And Ukrainian officials have warned other areas may have suffered worse than Bucha, including nearby Borodyanka.

"Locals talk about how planes came in during the first days of the war and fired rockets at them from low altitudes at these buildings," Ukraine's Interior Minister Denys Monastyrsky told local media.

Officials have alleged that Russian troops are now trying to cover up atrocities elsewhere to prevent further international outcry, including in the besieged city of Mariupol.

Ukrainian human rights official Lyudmila Denisova said on Telegram on Wednesday, citing witness testimony, that Russian forces have brought mobile crematoria to burn bodies and other heavy equipment to clear debris in the city.

Sanctions 'not enough'

Western powers have already pummelled Russia with debilitating economic sanctions, which forced Moscow on Wednesday to make foreign debt payments on dollar-denominated bonds in roubles, raising the prospect of a potential default.

On Wednesday, the White House unveiled further measures targeting Russia's top banks and two of President Vladimir Putin's daughters, while Britain sanctioned two banks and vowed to eliminate all Russian oil and gas imports by the end of the year.

The EU is also poised to implement a fifth round of sanctions cutting off Russian coal imports - and European Council chief Charles Michel said that "sooner or later", it must also impose oil and gas sanctions.

But in his nightly address, Zelensky said although the sanctions package had "a spectacular look... this is not enough." He urged countries to completely cut off Russia's banks from the international financial system, and to stop buying the country's oil.

"It is the export of oil that is one of the foundations of Russian aggression," he said.

"One of the foundations that allows the Russian leadership not to take seriously the negotiations on ending the war."

Peace talks between the sides have made little progress so far, and Nato chief Jens Stoltenberg has said there is no sign Putin has dropped "his ambition to control the whole of Ukraine".