Ukrainian President Volodymyr Zelensky announced Monday that Russia's long-awaited offensive in the Donbas region had begun, setting off a decisive second phase of the war centered on a battle for territory in eastern Ukraine.

The latest: Russian Foreign Minister Sergei Lavrov also said in an interview with India Today Tuesday that the Kremlin was beginning a new stage in its invasion of Ukraine, predicting that it would be "a very important moment."

• "Another stage of this operation [in eastern Ukraine] is beginning and I am sure this will be a very important moment of this entire special operation," Lavrov said.

Why it matters: Russian forces pulled back from Kyiv earlier this month to refocus their efforts on the Donbas. U.S. officials and analysts expect Russia to try to overwhelm and encircle Ukrainian forces after failing in its initial push to seize the capital.

What they're saying: "Now we can already state that the Russian troops have begun the battle for the Donbas, for which they have been preparing for a long time," Zelensky said in a video posted to Telegram.

• "A very significant part of the Russian army is now concentrated for that offensive. No matter how many servicemen get thrown there, we will fight, we will defend ourselves. We will do that every day," Zelensky vowed.
• The General Staff of the Armed Forces of Ukraine said in a statement on Tuesday morning that Russian forces "made an attempt to break through our defenses along nearly the entire frontline" in their push for control of the Donbas region while also pressing Mariupol.

The big picture: Ukraine has been battling Russian-backed separatists in the industrial heartlands of the Donbas for the last eight years, making it home to some of Kyiv's most elite fighters.

• The largely flat and wide-open terrain is expected to favor Russia, however, removing some of the challenges created by urban warfare and exploited by Ukrainian forces in the initial phase of the war.
• Russia has concentrated tanks and heavy artillery in the east in preparation for a grinding battle for territory, which Ukraine's foreign minister has warned will produce scenes reminiscent of World War II.
• Acknowledging the coming battle for the Donbas, President Biden authorized an additional $800 million in additional military aid to Ukraine last week that contained supplies of heavy weaponry that were absent from earlier packages.

Driving the news: Heavy shelling was reported Monday alongside nearly the entirety of the 300-mile frontline, including the Kharkiv, Donetsk and Luhansk regions, according to Oleksiy Danilov, the secretary of Ukraine's National Security and Defense Council.

• Russia claimed it hit hundreds of military supply targets around Ukraine in anticipation of the offensive, including in the western city of Lviv, where at least seven people were killed in missile strikes early Monday.
• In besieged Mariupol, the third-largest city in the Donbas, the last remaining Ukrainian forces were still holding out in a fortress-like steel plant under heavy bombardment from Russia.
• A senior U.S. defense official told reporters Monday that fully capturing Mariupol would allow Russia to free up 12 battalion tactical groups — units generally made up of 700-900 soldiers each — to be deployed elsewhere.

Go deeper: What to know about the coming battle for the Donbas

Editor's note: This article has been updated with comment from Ukraine's General Staff.