Russia launched its long-awaited all-out attack on east Ukraine on Tuesday, unleashing thousands of troops in what Ukraine described as the "Battle of the Donbas," a campaign to seize two provinces and salvage a battlefield victory.

Ukrainian officials insisted their troops would withstand the new attack, which they said began overnight with massive Russian artillery and rocket barrages and attempts to advance across almost the entire stretch of the eastern front.

In the first reported success of Russia's new attack, Ukraine said the Russians had seized Kreminna, a frontline town of 18,000 people in Luhansk, one of the two Donbas provinces.

"Kreminna is under the control of the 'Orcs.' They have entered the city," the province's Ukrainian governor, Sergiy Gaidai, told a briefing, invoking the goblin-like creatures who appear in J.R.R. Tolkein's fantasy books.

Russian forces are attacking "on all sides", and authorities are trying to evacuate civilians and it is impossible to tally the civilian dead, Gaidai said.

Moscow gave few details about its new campaign, but Foreign Minister Sergei Lavrov confirmed in an interview on Tuesday that the second phase of Russia's "special military operation" has begun, hoping to "completely liberate" the populations of Donetsk and Luhansk.

According to the Ukrinform news agency, up to 30 percent of Ukraine's infrastructure has been damaged since late February when Russia initiated the attack.

The hostilities have either destroyed or damaged 300 bridges and more than 8,000 km of roads, said Ukrinform, quoting Ukrainian Infrastructure Minister Oleksandr Kubrakov.

In the ruins of Mariupol, a southeastern port destroyed while withstanding nearly eight weeks of siege, Russia gave the last Ukrainian defenders holed up in a giant steel works an ultimatum to surrender by noon (0900 GMT) or die.

"All who lay down their arms are guaranteed to remain alive," the defense ministry said. The pro-Kremlin leader of Chechnya, whose forces have been fighting in Mariupol, predicted that troops would capture the plant on Tuesday.

President Volodymyr Zelenskyy told Ukrainians in a video address overnight that they would withstand the new advance.

"No matter how many Russian troops they send here, we will fight. We will defend ourselves," he said.

Driven back by Ukrainian forces in March from an attack on Kyiv in the north, Russia has poured troops into the east to regroup for a ground offensive in the Donbas. It has also been launching long-distance strikes at other targets including the capital.

(With input from Reuters, Xinhua)

(Cover: Ukrainian Armed Forces tanks ride along a street in a village in Donetsk Region, Ukraine, April 18, 2022. /Reuters)