President Volodymyr Zelenskyy said Ukraine is preparing for "important battles" against Russia's forces in the east of the country.
It comes as Pope Francis calls for an Easter truce to end the war.
Evacuations continued from Kramatorsk, in eastern Ukraine, where a missile strike on Friday killed 57 people at a railway station, according to a revised toll.
Meanwhile a strategic airport in the city of Dnipro was badly damaged in fresh shelling on Sunday.
President Volodymyr Zelenskyy again condemned atrocities against civilians, and after speaking with German Chancellor Olaf Scholz said they had agreed "that all perpetrators of war crimes must be identified and punished".
Ukraine's prosecutor general Iryna Venediktova said the country was examining the alleged culpability of 500 leading Russian officials for thousands of war crimes, including President Vladimir Putin.
And White House National Security Advisor Jake Sullivan pledged the US would "work with the international community to make sure there's accountability" for what he called "mass atrocities".
Pope Francis called for an Easter ceasefire in Ukraine to pave the way for peace.
Pope Francis at the Palm Sunday mass in St Peter's square on 10 April 2022. Source: ABACA / AAP
The pontiff denounced a war where "defenceless civilians" suffered "heinous massacres and atrocious cruelty".
But Russia's Orthodox Patriarch Kirill, a key Putin ally, issued his own appeal against Russia's "enemies".
"In this difficult period for our fatherland, may the Lord help each of us to unite, including around power," he said.
"This is how true solidarity will emerge in our people, as well as an ability to push back external and internal enemies, and to build a life with more good, truth and love.
'Ready to fight'
The remarks came after Zelenskyy said Ukraine was readying for a new Russian onslaught.
"We see the preparations for important battles, some people say decisive ones, in the east," he said on Saturday at a press conference with visiting Austrian Chancellor Karl Nehammer.
"We are ready to fight and to look in parallel to end this war through diplomacy."
Launching his own diplomatic initiative, Nehammer said he would meet Putin on Monday in a move his spokesman insisted was coordinated with "Berlin, Brussels and... Zelenskyy".
He will be the first European leader to visit the Kremlin since the invasion began on 24 February.
A satellite photo shows smoke rising in the minutes after a missile struck a train station in Kramatorsk, Ukraine on 8 April 2022. Credit: Planet Labs PBC/AAP
Pavlo Kyrylenko, the governor of Donetsk region, updated the death toll from Friday's attack at the station in Kramatorsk from 52 to 57 in a post to Telegram.
Ukraine's prosecutor Venediktova said 1,222 bodies had been found in the region around Kyiv alone so far.
At least two corpses were found inside a manhole at a petrol station on a motorway outside Kyiv on Sunday, an AFP reporter saw.
The bodies appeared to be wearing a mix of civilian and military clothing.
A distraught woman appeared at the manhole and peered inside, before breaking down and clawing the earth.
She wailed "my son, my son".
Ukraine said Kremlin propaganda had laid the groundwork for the bloody campaign, and that the Russian media had played its part.
"For many years, Russian political elites and propaganda have been inciting hatred, dehumanising Ukrainians, nurturing Russian superiority and laying ground for these atrocities," Foreign Minister Dmytro Kuleba tweeted on Sunday.
'Rockets keep flying'
The comments came on the heels of a surprise visit to Kyiv on Saturday by British Prime Minister Boris Johnson.
Hailing the country's response to the Russian invasion, Johnson offered Ukraine armoured vehicles and anti-ship missiles, crucial to halting the Russian naval siege of Black Sea ports, to help ensure that the country will "never be invaded again".
In a bid to shore up international resolve against Moscow, US President Joe Biden is to hold virtual talks on Monday with Indian Prime Minister Narendra Modi, just weeks after saying India had been "shaky" in its response to the invasion.
At the same time, EU foreign ministers are to meet to discuss a sixth round of sanctions, even as divisions over a ban on Russia gas and oil imports threatens to blunt their impact.
In further Russian strikes, the airport in Dnipro between central and eastern Ukraine, was hit hard.
"The airport itself and the infrastructure around it has been destroyed. Rockets keep flying and flying," the head of the city's military administration, Valentin Reznichenko, said on Telegram.
An AFP reporter saw black smoke in the sky above the facility, but a plane also took off later on Sunday, suggesting its runway was still functioning.
'New normal'
As Russian forces regroup in the east and south of Ukraine, local officials are urging residents to flee before it is too late.
Weekend bombardments in eastern Ukraine killed 10 civilians, including a child and wounded 11 others around Kharkiv, the region's governor Oleg Synegubov posted on Telegram on Sunday.
Earlier, on Facebook, he had reported 66 strikes over 24 hours.
"The Russian army continues to wage war on civilians due to a lack of victories at the front," he said.
A Ukrainian serviceman says farewell to his wife after she boards an evacuation train departing the Kramatorsk railway station. Credit: AAP
We evacuated "2,700-2,500 people per day, but now there are fewer and fewer," he said. "Today, only 200 people have been evacuated.
"I'm sure that 20-25 per cent" of the Lugansk region's population are still there, he added.
"Unfortunately, sometimes we just beg (them) to come out of hiding because we know what comes next..."
Russian forces, he said, "will destroy everything in their path, so for the sake of people, for the sake of their survival, we beg them to go anyway".
A Russian defence ministry statement meanwhile said the Kyiv authorities and their western allies continued to stage "monstrous and merciless" provocations and murder civilians in the self-proclaimed Lugansk People's Republic.
