Ukraine said Russia had started an anticipated new offensive in the east of the country, with explosions reported all along the front lines as well as attacks in other regions.
Ukrainian President Volodymyr Zelenskiy said Russia had begun the "Battle of Donbas" in the east on Monday 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 vowed in a video address.
Zelenskiy's chief of staff, Andriy Yermak, called it "the second phase of the war" and assured Ukrainians their forces could hold off the offensive. "Believe in our army, it is very strong," he said.
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. Ukrainian local officials and local media also said explosions were heard in Kharkiv in the northeast of Ukraine, Mykolaiv in the south and Zaporizhzhia in the southeast.
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" on Monday morning.
Driven back by Ukrainian resistance in the north, Moscow 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.
Russia's defence ministry said it had hit hundreds of military targets in Ukraine overnight.
Western capitals and Kyiv accuse Russian President Vladimir Putin of unprovoked aggression, and the White House said U.S. President Joe Biden would hold a call with allies on Tuesday to discuss the Ukraine 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 17 from the start of the Russian invasion on Feb. 24.
About 4 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 Kyiv says is evidence of atrocities, saying Ukraine has staged them to undermine peace talks.
