Russian forces attacked along a broad front in Eastern Ukraine on Tuesday as part of an offensive to take control of the country's eastern industrial heartland. 

Ukrainian officials are calling it a "new phase of the war." Russia invaded the country on Feb. 24.

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 defences along nearly the entire front line," the General Staff said in a statement early Tuesday.

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

"No matter how many Russian troops are driven there, we will fight," Zelensky vowed. "We will defend ourselves."

After its initial attempt to sieze Kyiv, the capital of Ukraine, failed, Russia has said its main goal is now to capture the Donbas. 

Separatists backed by Moscow have been fighting Ukrainian forces for eight years in the mostly Russian-speaking region. The separatists have declared two independent republics, which have been recognized by Russia.

Russian forces are reported to have made several gains, including the city of Kreminna. That brings the Russians closer to the city of Slovyansk, which was seized by pro-Russian fighters in 2014, but retaken by Ukrainian forces months later.