Ukraine's President Volodymyr Zelensky will press the UN Security Council on Tuesday for tougher sanctions aimed at forcing Russia to abandon its war against his country, as outrage grows against Moscow over the alleged slaughter of civilians.
Western officials have already promised new measures this week targeting Russia's oil and coal exports, which are helping President Vladimir Putin pay for the invasion he launched nearly six weeks ago.
And Denmark and Italy became the latest European nations to expel a group of Russian "intelligence officers" registered as diplomats, following France and Germany on Monday.
The moves follow the harrowing discoveries of scores of bodies in Bucha and other towns near Kyiv as Russian troops retreated to regroup in the east.
Most of the bodies have been found in civilian clothes. Ukrainian officials have said some had their hands bound behind their backs.
Zelensky has denounced "war crimes" and attempted "genocide" and appealed for more Western weapons and defence aid, saying they could have helped save innocent lives.
"The sanctions response to Russia's massacre of civilians must finally be powerful," he said in a Telegram video posted late Monday after touring the devastated streets of Bucha.
"But... did hundreds of our people have to die in agony for some European leaders to finally understand that the Russian state deserves the most severe pressure?" he asked.
His address to the UN Security Council will be the first since the invasion began on February 24th, though it was not clear if it would be pre-recorded or live.
In Bucha, resident Olena told AFP she saw Russian soldiers shoot a man in cold blood as units of "brutal" older troops sowed fear in the town near Kyiv.
"Right in front of my eyes, they fired on a man who was going to get food at the supermarket," said Olena, 43, who did not wish to give her second name.
EU Commission president Ursula von der Leyen will travel to Kyiv this week alongside EU foreign policy chief Joseph Borrell.
More weapons en route?
Many in Ukraine however are bracing for further Russian bombardments especially in the east and south, and air raid sirens rang out overnight across much of the country.
Russian authorities have denied the charges of civilian killings in Bucha and other locations near the capital, with its defence ministry on Tuesday claiming they had been staged by Ukraine.
"Similar events are now being organised by the Ukrainian special services in Sumy, Konotop and other cities," the Russian defence ministry said, referring to cities in northeastern Ukraine -- without offering evidence to back up the assertion.
But US President Joe Biden told reporters Monday that there should be "a war crimes trial" for Putin, vowing that Washington would join the EU in announcing new sanctions this week.
But Germany warned again that it was too soon to cut off purchases of Russian natural gas, a key source of Russian income that several nations have called for, but which would cripple large swathes of the European economy.
"At the moment, it's not possible to cut the gas supplies. We need some time," Finance Minister Christian Lindner said.
But US National Security Adviser Jake Sullivan warned Monday that the "next phase" of Russia's invasion, focusing on expanding territories it holds in the east and south, "could be measured in months or longer."
He said Western allies were working on obtaining more weapons for Ukraine, with Secretary of State Antony Blinken traveling to Brussels on Wednesday and Thursday to attend a NATO foreign ministers' meeting.
Horrors emerge
The full nature of the killings in Bucha and other areas from which Russian troops have withdrawn is still being pieced together.
On Monday, the bodies of five men were found in the basement of a children's sanatorium basement in Bucha. Ukraine prosecutors said they were unarmed civilians who had been bound, beaten and killed by Russian troops.
And in Motyzhyn, west of Kyiv, Ukrainian police showed AFP journalists the bodies of five people with their hands tied, including those of the village's mayor, her husband and son.
Ukrainian officials say over 400 civilian bodies have been recovered from the Kyiv region, many of whom have been buried in mass graves.
But Zelensky has warned that the deaths in Bucha could be only the tip of the iceberg, saying he had information that even more people had been killed in places like nearby Borodianka.
AFP reporters who briefly visited the area saw no bodies in the streets, but locals reported many deaths.
"I know five civilians were killed," said 58-year-old Rafik Azimov. "But we don't know how many more are left in the basements of the ruined buildings after the bombardments."
"I buried six people," another resident, Volodymyr Nahornyi, said. "More people are under the ruins."
Russians regroup
The Ukrainian government has warned that Moscow is preparing a "full-scale" attack in the country's east and regional officials urged civilians to evacuate Lugansk fearing a major Russian attack.
Even where troops have withdrawn, fears remain, with Kyiv's Mayor Vitali Klitschko telling residents to wait before trying to return, citing the danger of continued shelling and the danger of unexploded munitions.
On Monday, officials in Mykolaiv, on the Black Sea not far from Odessa, said cluster bombs were used against the city in strikes that killed 10 civilians and wounded 46.
A 2008 UN convention bans the production and use of cluster bombs, which kill indiscriminately by sending dozens of small bomblets over a large area, but it has not been signed by Russia or Ukraine.
Elsewhere in the south, concerns remain about civilians trapped in the besieged city of Mariupol, which has been besieged by Russian forces for over a month.
Authorities say at least 5,000 people have been killed in the city, 90 percent of which has been destroyed, according to Mayor Vadym Boichenko.
The Red Cross said Tuesday that Russian forces had released a team sent to help evacuate Mariupol residents that was detained en route Monday.
Europe's worst conflict in decades has killed as many as 20,000 people since Russia's February 24 invasion, according to Ukrainian estimates.
More than 4.2 million Ukrainians have fled the country and about 6.5 million have been internally displaced, UN agencies say.