Zelensky: 'Battle of Donbas' underway as Russians launch major offensive

April 18 (UPI) -- The second phase of Russia's war on Ukraine has begun with the launching of a widely anticipated offensive in the eastern Donbas region, Ukrainian President Volodymr Zelensky said Monday.

"Now we can say that Russian troops have started the battle for Donbas they have long been preparing for," Zelensky said in a video address, according to the official Ukrinform press service.

"A large part of the Russian army is now focused on this offensive," he said. "No matter how many Russian soldiers are driven there, we will fight. We will defend ourselves. We will do this every day.

"We will not give anything Ukrainian, and we do not need something that does not belong to us."

Zelensky said Russian invaders are acting with more strategic purpose in Donbas than previously, applying more pressure and actively seeking a weak spot in the country's defenses.

"Apparently, the Russian generals accustomed to not counting any losses have already killed so many Russian servicemen that even they have to be more careful. Otherwise, no one will be left to attack," Zelensky said.

"But, let them not hope that this will help them," he added. "It is just a matter of time when we liberate the entire territory of our state."

Zelensky's address came after Ukrainian national security spokesman Oleksiy Danilov tweeted that "an active phase of the Russian offensive began almost along the entire front line" in the Donetsk and Luhansk areas of the Donbas and in the northern Kharkiv region.

"Russian military was tasked to occupy the territories of the Donetsk and Luhansk regions by Easter, thus making a gift to their Fuhrer," he wrote. "However, they will not succeed!"

The Ukrainian military general staff also confirmed the start of a major offensive in the east with new troop build-ups forming in occupied Crimea and elsewhere, the Kyiv Independent reported.

There are now 76 Russian battalion tactical groups in Donbas, with about 11 of those added during the last several days, a senior U.S. Defense Department official said Monday.

The heaviest fighting in Donbas is around the town of Popasna, with fighting also continuing to the south and the southeast of the Ukrainian city of Izyum, the unnamed Pentagon official said in a background briefing.

About a dozen of the Russian tactical groups now inside Ukraine are trying to take strategic southern port city of Mariupol, which still remains in Ukrainian hands, the official said.

Pentagon spokesman John Kirby said it remains uncertain how long Ukrainian forces can hold out there.

"We've seen no sign out of the Ukrainians that they're not willing to continue to defend that city," Kirby told CNN.