Ukraine's foreign minister says Russia has repeatedly violated its own three-day ceasefire, even hours after it began, and called the initiative a "farce", while Moscow says Kyiv had continued fighting.
There was, though, a drop-off in combat activity after the ceasefire announced by Russian President Vladimir Putin came into force in the early hours of Thursday, with a respite in the drone and missile attacks that had rattled Ukrainian cities earlier this week.
"Predictably, Putin's 'Parade ceasefire' proves to be a farce," Ukrainian Foreign Minister Andrii Sybiha said on X, referring to the truce which coincides with a May 9 parade on Moscow's Red Square to commemorate the end of World War Two.
"Russian forces continue to attack across the entire frontline," Sybiha wrote. "From midnight to midday, Russia committed 734 ceasefire violations and 63 assault operations, 23 of which are still ongoing."
He said Kyiv was notifying the United States and European states about Russia's actions.
The Russian defence ministry, cited by Interfax news agency, said that Ukraine, in turn, had carried out 488 attacks on Russian targets and twice tried to break through the border in the Kursk region.
The two sides did not immediately comment on each other's battlefield reports, which Reuters could not independently confirm.
A late night report issued by the General Staff of Ukraine's military nearly 24 hours in the proclaimed ceasefire said 154 clashes had been recorded. Russian forces, it said, had launched one missile attack and 15 air strikes.
The governor of southeastern Zaporizhzhia region said a Russian drone had struck a car in the south of the region, killing a passenger.
In central Poltava region, the head of the regional military administration said air defence units had downed a missile deployed by Russian forces. The missile damaged private homes, but caused no casualties.
Ukraine has not committed to abide by the ceasefire, calling it a ruse by Putin to create the impression he wants to end the war, which began when Russia launched a full-scale invasion of Ukraine in February 2022. Putin says he is committed to achieving peace.
Reuters journalists with a Ukrainian drone unit near the front in eastern Ukraine said a small Russian infantry raiding party had tried to advance on Thursday, but had been stopped by drones piloted by members of the unit.
Ukrainian soldiers observed the clash on a live feed streamed onto monitors in their bunker.
"The infantry are still coming," said one of the soldiers in the unit, a 33-year-old who identified himself by his callsign, "Mikha."
A second person in the same unit, who identified himself as Nazar, said in the six hours since the Russian ceasefire started, there had been three Russian strikes on his section of the front. Asked if the ceasefire was holding, he said: "The facts speak for themselves."
A Ukrainian military spokesman earlier said Russia had continued assaults in areas on the eastern front and prosecutors said two people had been wounded along with the 55-year-old woman killed by bombs fired at the northern Sumy region.
The Russian ceasefire falls on the 80th anniversary of the World War Two defeat of Nazi Germany, for which Putin is hosting Chinese President Xi Jinping and other leaders ahead of a military parade on Red Square on May 9.
Ukraine, like the West, marks the anniversary on May 8.
The criticism comes after Ukrainian President Volodymyr Zelensky said he told US President Donald Trump in a telephone call that a 30-day ceasefire would be a "real indicator" of moving towards peace with Russia and Kyiv was ready to implement it immediately.
Zelensky marked the day by taking a rare walk in central Kyiv to pay his respects to fallen Ukrainian soldiers at a vast mound of Ukrainian flags planted on a grassy verge on the central square.
There was no sign of his security detail in the selfie video he filmed as he passed pedestrians on the city's main drag, at one point pausing to say "hi" as cars tooted their horns and deriding Friday's planned pomp-filled ceremony in Moscow.
"There will be a parade of cynicism. You simply cannot call it anything else. A parade of bile and lies. As if not dozens of allied states, but Putin personally defeated Nazism," he said.
Ukraine launched successive drone attacks on Moscow this week, which had forced the closure of airports in the Russian capital and the grounding of airliners.
Russia's aviation authority Rosaviatsiya said on Thursday evening that airline schedules, heavily disrupted earlier in the week, were now operating normally.
Zelensky spoke to Trump in the middle of a three-day ceasefire proclaimed by Russian President Vladimir Putin, coinciding with commemorations of the 80th anniversary of the World War Two victory over Nazi Germany.
Zelensky has dismissed the three-day pause as meaningless and each side has accused the other of violating it.
"Ukraine is ready for a complete 30-day ceasefire from this very day, from this very moment," Zelensky said in his nightly video address.
"But it has to be real... Thirty days which could become the beginning of years of peace. A ceasefire, both prolonged and reliable, will be a real indicator of a movement towards peace."
The Ukrainian president said Russia had to demonstrate its readiness to end the war, starting with an unconditional ceasefire.
Trump, in a social media post on Thursday, again called for a 30-day unconditional ceasefire, though he made no reference to his conversation with Zelensky.
He said that if the 30-day ceasefire "is not respected, the US and its partners will impose further sanctions."
A 30-day ceasefire was initially proposed by Washington in March and Ukraine agreed. Russia has said such a measure can only take effect once reliable measures of monitoring and upholding it are put in place.
Zelensky said he also spoke to Trump about the ratification by Ukraine's parliament on Thursday of an agreement to exploit Ukraine's mineral riches along with the creation of an investment fund for Ukraine's reconstruction.
He said he and Trump "noted how important it is that our relationship has strengthened our countries over the decades".
Should peace talks fail, Germany's new Chancellor Friedrich Merz assured Zelensky that his government could be relied upon for continued support against Russia's invasion, in a telephone call, a government spokesperson said.
Merz, who took office earlier this week after his conservatives won a February federal election, told Zelensky Germany supports the mediation efforts of US President Donald Trump, in close coordination with other European partners.
"This includes helping Ukraine defend itself effectively against Russian aggression and exerting pressure on Russia," the spokesperson said.
Zelensky, in his nightly video address, said he and Merz agreed to work together on matters of joint interest.
"There are already concrete things that we can do together," Zelensky said. "Decisions will be taken."