Ukraine urged its residents in the east of the country Thursday to take their "last chance" to flee mounting Russian attacks, after devastation around the capital Kyiv shocked the world.

Six weeks after they invaded, Russian troops have withdrawn from Kyiv and Ukraine's north and are focusing on the country's southeast, where desperate attempts are under way to evacuate civilians.

The retreat from Kyiv revealed scenes of carnage, including in the town of Bucha, that Ukraine said were evidence of Russian war crimes, and which triggered a fresh wave of Western sanctions against Moscow.

On Thursday, Ukrainian President Volodymyr Zelensky warned that Russia -- which denies responsibility for the killings of civilians -- was undeterred and continued "to accumulate fighting force to realise their ill ambitions in (eastern) Donbas".  

"They are preparing to resume an active offensive," he said, while officials in Donbas' Lugansk and Donetsk regions begged civilians to leave.

"These few days may be the last chance to leave," Lugansk regional governor Sergiy Gaiday wrote on Facebook, saying that all cities in the region were under fire and one person had died in the town of Kreminna.

"Do not wait to evacuate," he said, adding: "The enemy is trying to cut off all possible ways of getting people out."

Nowhere to go

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

For those 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". 

Large areas of Lugansk and the neighbouring Donetsk region have been controlled since 2014 by pro-Russian separatists.

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 Severodonetsk.

More than 11 million people have been displaced since Russia invaded on February 24, with the stated aim to "demilitarise" Ukraine and support Moscow-backed separatists.

It is currently believed to be trying to create a land link between occupied Crimea and the statelets in Donbas.

Weapons, weapons, weapons

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 Kyiv's Foreign Minister Dmytro Kuleba on Thursday made a fresh appeal to NATO 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 President Vladimir 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. 

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

"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," 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 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 and on Wednesday the United States unveiled further measures targeting Russia's top banks and two of Putin's daughters.

Britain sanctioned two banks and vowed to eliminate all Russian oil and gas imports by the end of the year, while the European Union is poised to cut off Russian coal imports.

EU nations this week have also expelled more than 200 Russian diplomats and staff, while a vote will be held later Thursday in the UN General Assembly on excluding Moscow from the UN Human Rights Council.

"We are convinced that now is the time to suspend Russian membership of the Human Rights Council," G7 foreign ministers from Canada, France, Germany, Italy, Japan, Britain and the United States said in a statement.

But in his nightly address, Zelensky said the new sanctions were "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. 

Oil exports are "one of the foundations of Russian aggression", he said, which "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".