Russian forces have launched their anticipated offensive in eastern Ukraine, attempting to push through defences along almost the entire front line early on Tuesday in what Ukrainian officials described as the second phase of the war.
Ukrainian president Volodymyr Zelenskiy said Russia had begun the "Battle of Donbas" in the east and a "very large part of the entire Russian army is now focused on this offensive".
"No matter how many Russian troops they send there, we will fight. We will defend ourselves," he said in a video address on Monday.
Mr Zelenskiy's chief of staff, Andriy Yermak, assured Ukrainians their forces could hold off the offensive in "the second phase of the war".
"Believe in our army, it is very strong," he said.
There was no immediate comment from Russia's defence ministry on the latest fighting. The governor of the Russian province of Belgorod said Ukrainian forces had struck a border village wounding one resident.
Ukrainian media reported a series of explosions, some powerful, along the front line in the Donetsk region, with shelling taking place in Marinka, Slavyansk and Kramatorsk.
Blasts were also heard in Kharkiv in the northeast, Mykolaiv in the south and Zaporizhzhia in the southeast while air raid sirens were also going off in main centres near the front line, officials and media said.
Reuters was not immediately able to verify the reports.
Ukraine's top security official, Oleksiy Danilov, said Russian forces attempted to break through Ukrainian defences "along almost the entire front line of Donetsk, Luhansk and Kharkiv regions".
Driven back by Ukrainian forces in the north, Russia has refocused its ground offensive in the two eastern provinces known as the Donbas, while launching long-distance strikes at other targets including the capital, Kyiv.
Donbas has been the focal point of Russia's campaign to destabilise Ukraine, starting in 2014 when the Kremlin used proxies to set up two separatist "people's republics" in the ex-Soviet state. It is also home to much of Ukraine's industrial wealth, including coal and steel.
Ukraine's general staff said Russian forces aimed to establish full control over the Donetsk, Luhansk and Kherson regions, while intensifying missile strikes in west Ukraine.
Biden to host call with allies
Western countries and Ukraine accuse Russian president Vladimir Putin of unprovoked aggression, and the White House said US president Joe Biden would hold a call with allies on Tuesday to discuss the crisis, including on how to coordinate on holding Russia accountable.
French president Emmanuel Macron said his dialogue with Putin had stalled after mass killings were discovered in Ukraine.
The United Nations said on Monday the war's civilian death toll had surpassed 2,000, reaching 2,072 as of midnight on April 17th from the beginning of the invasion on February 24th.
About four million Ukrainians have fled the country.
Russia denies targeting civilians in what it calls a special operation to demilitarise Ukraine and eradicate dangerous nationalists. It rejects what Ukraine says is evidence of atrocities, saying Ukraine has staged them to undermine peace talks.
'Hell on Earth'
Russia has been trying to take full control of the southeastern port city of Mariupol, which has been besieged for weeks and which would be a big strategic prize, linking territory held by pro-Russian separatists in the east with the Crimea region that Moscow annexed in 2014 and freeing up the besieging troops.
Video footage showed block after residential block in charred ruins. Shell-shocked residents in the Primorskyi district cooked on open fires outside their damaged homes.
"To be honest, we are not well," one resident named Olga told Reuters. "I have mental problems after air strikes, that's for sure. I'm really scared. When I hear a plane I just run."
The city council said at least 1,000 civilians were still hiding in shelters beneath the vast Azovstal steel plant, which contain myriad buildings, blast furnaces and rail tracks.
Major Serhiy Volyna, commander of Ukraine's 36th marine brigade which is still fighting in Mariupol, appealed for help in a letter to Pope Francis.
"This is what hell looks like on earth ... It's time (for) help not just by prayers. Save our lives from satanic hands," he said in the letter, according to excerpts that Ukraine's Vatican ambassador posted on Twitter.