(NewsNation) — A renewed fight for the eastern region of Ukraine appears to be underway with a rethought strategy from Russia, Pentagon spokesperson John Kirby told NewsNation on Monday, echoing an update from Ukrainian President Volodymyr Zelenskyy.

“The Russian troops have begun the battle for the Donbas,” Zelenskyy announced in a video address. He said a “significant part of the entire Russian army is now concentrated on this offensive.”

Kirby told NewsNation’s Kellie Meyer the Pentagon has observed a switch in Russia’s strategy. Logistical pipelines for food and equipment appear to already be in place for this theater of the war, he said. When Russia tried to take Kyiv, they expected it would go so quickly that those infrastructure chains could form later.

Kyiv did not fall, and the U.S. believes that’s partly because the 40-mile convoy of Russian tanks stalled due to a lack of supplies. Kirby says it appears they’ve learned from that mistake.

But Kirby also said it’s “too soon to tell” if this attack will go any better for Russia than their failed attempt to siege Ukraine’s capital..

“It doesn’t mean they fixed all their logistics problems or integrated air and ground operations,” Kirby said. “It remains to be see how much this effort will bear fruit for the Russians.”

The Donbas region in eastern Ukraine is home to more Russian sympathizers than other parts of the country. There’s been a simmering war there for eight years, and Putin began the war after alleging that separatists there asked for his help.

“The operation [Russia’s] been doing for a decade have allowed them to put in place logistics, communication networks, map out the areas and understand the road networks and understand where the opportunities for Russians are to get in and how to cut off the options for Ukrainians to mount a counter offensive,” Brian Clark, senior fellow at the Hudson Institute, said Monday on “NewsNation Prime.”

Ukrainian MP Inna Sovsun said that doesn’t make a Russian victory inevitable.

“We do realize it was being prepared for quite a long period of time,” she told NewsNation. “But we have been preparing, too.”

Though some have speculated the fighting could stop if Ukraine ceded that part of the country to Putin, Zelenskyy and others have vowed not to do so.

“No matter how many Russian troops are driven there, we will fight,” Zelenskyy said. “We will defend ourselves. We will do it every day.”

Because of the long conflict there, both sides are familiar with the terrain. Kriby said it is “a bit like Kansas — flat and open.”

The Pentagon believes the geography lends itself to more long-range attacks, which Kirby says highlights the need for artillery rounds.

President Joe Biden’s most recent $800 million aid package includes howitzer cannons, but they have not arrived in Ukraine yet. Kirby would not say when they would get there, only that the U.S. has secured them and they should make it to Europe “soon.” Americans will also provide training on the weapons outside Ukraine.

Despite the aid, Kirby said the next phase of this war will not be any easier for the world to watch.

“We think it could potentially be bloody and it could potentially be prolonged.”