An attack on a frontline civilian convoy killed at least 25 people in southern Ukraine on Friday, just hours before Moscow was due to annex four occupied Ukrainian regions.
Russian President Vladimir Putin is poised to formally annex Kherson, Zaporizhzhia, Donetsk and Luhansk, which his forces mostly control, at a grand Kremlin ceremony later on Friday.
He has warned he could use nuclear weapons to retain control the territory as the United States leads Western allies in vowing "never" to recognize the regions as anything other than part of Ukraine.
Kremlin spokesman Dmitry Peskov told reporters that the annexations would be formalized at an event at (12:00 GMT) during which Putin would deliver a "major" speech.
But early on Friday, an attack in Zaporizhzhia in the south, killed at least 25 people as civilians were preparing to leave to pick up relatives, Ukrainian officials said.
"Twenty-five killed and about 50 wounded in an attack by the Russian military on a humanitarian convoy in Zaporizhzhia. Investigation launched," said the prosecutor general's office on Telegram.
Bodies of people wearing civilian clothes were left on the ground after the attack and windows of cars blown out, an AFP photographer said.
"Only complete terrorists could do this," said Ukrainian President Volodymyr Zelensky. "Bloodthirsty scum! You will definitely answer," he added.
Some 20% of Ukraine
"The enemy launched rockets on a civilian convoy leaving the city center," said Zaporizhzhia governor Oleksandr Starukh.
"Rescuers, medics — all relevant services are currently working at the site," he added.
But pro-Kremlin regional chief Vladimir Rogov accused Ukrainian troops of carrying out a "terrorist act."
"The regime in Kyiv is trying to portray what happened as shelling by Russian troops, resorting to a heinous provocation," he said on social media.
Preparations were underway in Moscow's Red Square for state-organized celebrations to announce the annexation of Zaporizhzhia and the three other regions.
Municipal workers were climbing scaffolding in advance to install huge banners emblazoned with: "Donetsk. Luhansk. Zaporizhzhia. Kherson. Russia!"
The four create a crucial land corridor between Russia and the Crimean peninsula, annexed by Moscow in 2014.
Together, all five make up around 20% of Ukraine, whose forces in recent weeks have been clawing back ground as part of a sweeping counter-offensive.
It comes after Putin announced a draft to prop up Russian forces — leading to some protests and an exodus of men.
In what appeared to be a bid to calm discontent, Putin on Thursday called for mistakes with the draft to be "corrected."
In Kherson on Friday, Russian officials announced that Ukrainian strikes with US-supplied precision artillery systems had killed a senior security chief of the Russian-controlled region.
A "pinpoint" strike by Himars hit his house, Kirill Stremousov, the deputy head of the Russian proxy administration said.
That announcement was just the latest of several targeted attacks on Russian-appointed officials in the region where Ukrainians are clawing back territory.
Ukrainian forces are also on the doorstep of Lyman in Donetsk, which Moscow's forces pummelled for weeks to capture this summer.
"Lyman is partially surrounded," said Denis Pushilin, the pro-Moscow leader in the breakaway region of Donetsk, on social media. Two nearby villages were "not fully under our control," he added.
On Thursday, President Joe Biden said the United States would "never, never, never" recognize Russian sovereignty over the territories set for annexation.
UN Secretary-General Antonio Guterres condemned the annexation plans as "a dangerous escalation" that "has no place in the modern world."
Security Council vote
Britain's Prime Minister Liz Truss also said the U.K. would "never accept" the four regions "as anything other than Ukrainian territory."
Putin has blamed the conflict on the West and said conflicts in the former Soviet Union were the result of its collapse.
He has long considered the fall of the U.S.S.R. as a tragedy, and he has argued Moscow should again extend its influence over the former Soviet region.
The four regions' Kremlin-installed leaders this week assembled in the Russian capital ahead of the ceremony.
They formally requested annexation after claiming residents backed the move in hastily organized referendums that were dismissed by Kyiv and the West as fraudulent.
Ukraine has said the West's only appropriate response is to hit Russia with more sanctions and to supply Ukrainian forces with more weapons to keep reclaiming territory.
The UN Security Council will vote Friday on a resolution condemning the referendums, according to France, the council's current president, but it has no chance of passing due to Moscow's veto power.
Zelensky is calling an "urgent" meeting of his national security council for Friday, his spokesman said.