Ukrainian troops have repulsed several Russian assaults in the country's east, British intelligence said on Monday, while President Volodymyr Zelenskyy said thousands of Russian soldiers were massing for a new offensive.
Russian forces were also pushing to establish control over the southern port city of Mariupol, the lynchpin between Russian-held .
"There are tens of thousands of dead, but even despite this, the Russians are not stopping their offensive," Mr Zelenskyy told South Korea's parliament by videolink.
News agency Reuters could not verify the accuracy of his estimate.
Meanwhile, Russia claims to have used cruise missiles to destroy S-300 anti-aircraft missile systems which had been supplied to Ukraine by an unidentified European country.
Russia launched Kalibr cruise missiles on Sunday against four S-300 launchers which were concealed in a hangar on the outskirts of the Ukrainian city of Dnipro, the defence ministry said on Monday.
Russia said 25 Ukrainian troops were hit in the attack.
"High-precision sea-launched Kalibr missiles destroyed the equipment of a S-300 anti-aircraft missile division which had been delivered to the Kyiv regime by a European country," the ministry said.
Russia did not say which European country had supplied the S-300 systems.
Russian forces also shot down two Ukrainian Su-25 aircraft near the city of Izium and destroyed two ammunition depots, one of which was near the southern city of Mykolaiv, the Russian defence ministry said.
The Ukrainian military did not respond to a request for comment.
Damage is seen in Andriivka, a village in Ukraine. Source: AAP / SOPA Images/Sipa USA
About a quarter of Ukraine's 44 million population have been forced from their homes, cities turned into rubble, and thousands of people have been killed or injured — many of them civilians.
Austrian leader Karl Nehammer planned to meet Russian President Vladimir Putin in Moscow on Monday and will call for an end to the conflict. It would be Mr Putin's first face-to-face meeting with a European Union leader since
Russian forces have abandoned their attempt to capture the capital Kyiv, for now at least, but are redoubling their efforts in Ukraine's east. Britain's defence ministry said Russian shelling continued in the Donetsk and Luhansk regions.
But Ukrainian forces had beaten back several assaults and destroyed Russian tanks, vehicles and artillery equipment, it said in its regular intelligence bulletin.
Powerful explosions rocked cities in the south and east and air raid sirens blared out across Ukraine early on Monday.
'We will answer'
Mr Zelenskyy kept up his campaign to generate international support and rally his countrymen, warning the coming week would be important.
"Russian troops will move to even larger operations in the east of our state. They may use even more missiles against us, even more air bombs. But we are preparing for their actions. We will answer," he said in a late-night video address.
Addressing South Korea's parliament, he said Russia was concentrating tens of thousands of soldiers for the next offensive. He asked Seoul for any military aid it could provide.
Since Russia invaded, Mr Zelenskyy has appealed to Western powers to provide more defence help, and to punish Moscow with tougher sanctions including embargoes on its energy exports.
Mr Zelenskyy also said Mariupol has been destroyed. Reuters journalists on Sunday saw several Russian tanks heading down a highway in the direction of the city.
The general staff of Ukraine's armed forces said it was likely the Russians would try to disrupt supply lines and strike at transport infrastructure.
Ukrainian President Volodymyr Zelenskyy addresses South Korea's parliament via video link at the National Assembly in Seoul, South Korea, on 11 April, 2022. Source: AAP, EPA / Chung Sung-Jun
War 'must stop'
Austrian Chancellor Karl Nehammer said he would meet Mr Putin on Monday in Moscow.
"We are militarily neutral, but (have) a clear position on the Russian war of aggression against #Ukraine," Mr Nehammer wrote . "It must stop! It needs humanitarian corridors, ceasefire & full investigation of war crimes."
Mounting civilian casualties have triggered widespread international condemnation and new sanctions.
Ludmila Zabaluk, head of the Dmytriv Village Department, north of Kyiv, said dozens of civilian bodies were found in the area.
"There were more than 50 dead people. They shot them from close distance. There's a car where a 17-year-old child was burned, only bones left. A woman had half her head blown off. A bit farther, a man lying near his car was burned alive," she said.
Reuters could not confirm the reports.
Moscow has rejected accusations of war crimes by Ukraine and Western countries. It has repeatedly denied targeting civilians in what it calls a "special operation" to demilitarise and "denazify" its southern neighbour. Ukraine and Western nations have dismissed this as a baseless pretext for war.
German Foreign Minister Annalena Baerbock, speaking before a meeting of European ministers in Luxembourg on Monday, said Berlin saw "massive indications" of war crimes in Ukraine.
Number of refugees escaping Ukraine war exceeds 4.5 million: UN
More than 4.5 million Ukrainian refugees have fled their country since the Russian invasion on 24 February, according to the United Nations refugee agency.
The UNHCR said there were 4,503,954 Ukrainian refugees on Sunday. That was 62,291 more than the previous day.
Europe has not seen such an exodus of refugees since World War Two.
Ninety per cent of those who have fled Ukraine are women and children, as the Ukrainian authorities do not allow men of military age to leave.
According to the UN's International Organisation for Migration (IOM), around 210,000 non-Ukrainians have also fled the country, sometimes encountering difficulties returning to their home countries.
A further 7.1 million people have been displaced within the country, according to figures published by the IOM on 5 April.
That means more than a quarter of the population have been forced to flee their homes, seeking refuge either abroad or elsewhere in Ukraine.
Before the conflict, Ukraine was home to more than 37 million people in territory controlled by Kyiv — which does not include Crimea, annexed by Russia in 2014, or areas in the east under the control of pro-Russian separatists.
