(NewsNation) — Russian forces attacked along a broad front in eastern Ukraine on Tuesday as part of a full-scale ground offensive to take control of the country’s eastern industrial heartland in what Ukrainian officials called a “new phase of the war.”

Ukraine’s General Staff said Russian forces are focusing their efforts on taking full control of the Donbas region. “The occupiers made an attempt to break through our defenses along nearly the entire frontline,” the General Staff said in a statement.

The new assaults began Monday along a front of more than 300 miles focused on the Donbas regions of Donetsk and Luhansk, with the Russian forces trying to advance in several sections, including from the neighboring Kharkiv region.

Kharkiv’s region Governor Oleh Synyehubov said that five residents were killed and another 17 were wounded in the Russian shelling in the city on Tuesday.

In southern Donetsk, the General Staff said the Russian military has continued to blockade and shell Mariupol.

Ukrainian President Volodymyr Zelenskyy said in a video address Monday that a “significant part of the entire Russian army is now concentrated on this offensive.”

Moscow-backed separatists have been fighting Ukrainian forces for eight years in the mostly Russian-speaking Donbas and have declared two independent republics that have been recognized by Russia. Russia has declared the capture of the Donbas to be its main goal in the war since its attempt to seize the capital, Kyiv, failed.

“No matter how many Russian troops are driven there, we will fight,” Zelenskyy vowed. “We will defend ourselves.”

Troops fought in the streets of Kreminna on Monday before Russia was able to gain control of the city, according to Serhiy Haidai, Luhansk regional military administrator. Haidai said that before advancing, Russian forces “just started leveling everything to the ground.” He said his forces retreated to regroup and keep fighting.

The breakthrough at Kreminna brings the Russians closer to the city of Slovyansk, which is seen as a key target in the Russian offensive. Slovyansk was seized by pro-Russian fighters in 2014, only to be retaken by Ukrainian forces months later following intense fighting.

Russian troops have already seized the city of Izyum, which sits along a highway north of Slovyansk, and they are poised to push toward the city from the north and the east. Slovyansk lies just north of another key city, Kramatorsk, where an earlier Russian attack on a train station killed more than 50 people.

On Monday morning, Oleksiy Danilov, secretary of Ukraine’s national security council, told Ukrainian media that the defensive line had not been broken elsewhere.

“Fortunately, our military is holding out,” Danilov said.

In Mariupol, Denys Prokopenko, commander of the Azov Regiment of the Ukrainian National Guard, said that Russia had begun dropping bunker-buster bombs on the Azovstal steel plant where the regiment was holding out. The plant contains a warren of tunnels where both fighters and civilians are sheltering. It is believed to be the last major hub of resistance in the shattered city.

Russia has Mariupol surrounded and has been fighting a bloody battle to seize it since the beginning. If Russia takes Mariupol, it would free up troops for use elsewhere in the Donbas, deprive Ukraine of a vital port and complete a land bridge between Russia and the Crimean Peninsula, seized from Ukraine from 2014.

In western Ukraine near the Polish border, at least seven people were reported killed Monday in missile strikes on Lviv. Lviv has been a haven for civilians fleeing the fighting elsewhere. And to the Kremlin’s increasing anger, the city has also become a major gateway for NATO-supplied weapons.

The attack hit three military infrastructure facilities and an auto shop, according to the region’s governor, Maksym Kozytskyy.

Also, a hotel sheltering Ukrainians who had fled the fighting in other parts of the country was badly damaged, Lviv Mayor Andriy Sadovyi said.

“The nightmare of war has caught up with us even in Lviv,” said Lyudmila Turchak, who fled with two children from Kharkiv in the east.

Moscow said its missiles struck military targets in eastern and central Ukraine including ammunition depots, command headquarters and groups of troops and vehicles. It reported that its artillery hit hundreds of Ukrainian targets, and that warplanes conducted 108 strikes on troops and military equipment. The claims could not be independently verified.

Gen. Richard Dannatt, a former head of the British Army, told Sky News that Russia was waging a “softening-up” campaign ahead of the Donbas offensive.

A senior U.S. defense official, speaking on condition of anonymity to discuss the Pentagon’s assessments of the war, said there are now 76 Russian combat units, known as battalion tactical groups, in eastern and southern Ukraine, up from 65 last week. That could translate to around 50,000 to 60,000 troops, based on what the Pentagon said at the start of the war was the typical unit strength of 700 to 800 soldiers.