Ukraine on Thursday accused Russia of deliberately targeting civilians after 30 people were killed when two buses were blown up in the frontline city of Kramatorsk, in one of the deadliest attacks in weeks.
President Volodymyr Zelensky vowed that it "will not go unanswered" after photos posted by local authorities showed the burnt-out metal shell of the vehicles, with bodies and debris strewn across the road.
"Russia is trying to destroy all life in (the) Donbas (region) and the world must respond to this," he wrote on Telegram, warning the death toll could rise.
US Secretary of State Marco Rubio warned that the current stalemate after nearly five years of fighting was "very dangerous" and could lead to escalation.
Later Thursday, Zelensky said Ukrainian negotiators were headed to the United States for talks Friday and Saturday with a US team.
"We are also counting on a European team to join our negotiating group," he added.
The Kramatorsk strikes came just a day after five young children were among 22 killed when a Russian missile struck a five-storey apartment block in Pryluky, in the northern Chernigiv region.
UN rights chief Volker Turk condemned the wave of Russian attacks on civilians.
"I am appalled by the wave of attacks killing civilians across Ukraine over the past four days, including today's horrifying strike hitting two public buses.
"The scale and manner of these attacks -- killing and injuring people on buses, at their homes and work in different areas of the country -- is reprehensible."
"The Russians are deliberately targeting civilians," Donetsk governor Vadym Filashkin said, accusing Moscow of flouting "every conceivable rule of warfare and norm of international humanitarian law".
Kramatorsk City Council insisted there were "absolutely no military targets" in the area.
Three more people were killed in shelling at a nearby village, the emergency services said.
There has been mounting concern about the increase in civilian deaths and injuries as both sides step up long-range attacks.
The United Nations has verified more than 17,000 civilian deaths but says the real number is considerably higher.
Turk, in his statement Thursday, said both sides "must take constant care to spare the civilian population".
At least 534 people have died in Kyiv since the 2022 invasion, according to the city state administration's data seen by AFP on Thursday.
Last week, Zelensky said his Russian counterpart Vladimir Putin had sanctioned massive attacks on civilian areas, which the Kremlin denies.
But touring the site of the Pryluky attack on Thursday, visiting UK foreign minister Ed Miliband said there was "no clearer sign of Putin's evil, evil intent and strategy in this conflict".
Zelensky urged the United States to increase pressure on Russia, including through providing better air-defence systems.
Ukraine's capital Kyiv has come under increasing attack from a barrage of Russian ballistic and cruise missiles, and fast-moving, jet-powered drones.
For the first few years of the conflict, Kyiv had been relatively spared from the devastation wrought on the frontline.
But key transport bridges and the rail network have now been hit and rolling power cuts introduced after energy infrastructure was targeted.
Russia claims the eastern Donetsk region as its own and the area has been the epicentre of fighting but it has not yet captured Kramatorsk.
The city, which had a pre-war population of some 150,000, has been used as a logistics and supply hub for the Ukrainian military and is part of the so-called "fortress belt" which troops are fighting to retain.
Russian forces have edged close to the city's eastern fringes, which has prompted calls for the civilians who remain to relocate to safer regions of Ukraine.
Ukraine last month launched a counter-offensive against Russian troops around Lyman -- to the north of Kramatorsk and neighbouring Sloviansk -- claiming some success in regaining pockets of territory.
In the first strike of its kind in Russia, drone attack caused a fire at a data centre belonging to Russian technology giant Yandex, the company told AFP Thursday.
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