- Russian forces are already resorting to "indiscriminate" heavy weaponry such as artillery to attack Ukrainian cities.
- Russia focuses its military training on operations in open terrain, whereas the Ukrainians have long been preparing themselves for a door-to-door fight in the cities themselves.
- Kyiv's fate may hinge on the Ukrainians' ability to get more weapons from the West and halt a huge Russian convoy approaching from the north.
The next phase of the war in Ukraine is likely to exact an awful cost as Russia turns to bigger, more indiscriminate weapons to bombard cities and prepares for brutal urban fighting.
Despite fierce Ukrainian resistance, cities are being encircled, and Russian forces are already resorting to weapons such as artillery to bombard them, retired U.S. Army Col. Jack Jacobs told CNBC's "Squawk Box Asia."
"The Russians are going to increase their indirect fire on population centers, particularly on Kyiv," Jacobs said. Indirect fire refers to weapons that don't require a direct line of sight on the target. Such weapons, such as artillery, can used in large numbers to destroy large areas.
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Such an approach is one the Russians "didn't want to do, because they wanted to take the city intact," Jacobs said. Regardless, they'll use rockets, "artillery, missiles and other indirect fire in order to subdue the Ukrainians in the city. And then try to move in."
At that point, the war switches from one that plays to Russian military strengths to one that plays to the strengths of the Ukrainians. But it'll also be a more deadly war, because it will be fought up close within urban areas.
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Russia's military culture focuses training on operations in open terrain, Jacobs said, whereas the Ukrainians — including many of its civilians — have long been preparing themselves for a door-to-door fight in the cities themselves.
The Ukrainians "have known from the very beginning that ultimately, it may come down to their ability to destroy Russian forces inside the built-up areas," said Jacobs, who experienced urban fighting as an officer in the Vietnam War.
'Enormous cost'
Unfortunately, that scenario would come at a horrific cost to Ukraine's cities, which are still full of civilians although many have fled.
John Spencer, an expert on urban warfare at the Modern War Institute at the U.S. Military Academy, told NBC News that once Russian soldiers are in the cities in large numbers, "it's going to get really ugly."
"The Russians won't be able to take Kyiv by just bombing it," Spencer told NBC. "They'll have to get soldiers in there."
Russian President Vladimir Putin has already demonstrated a personal willingness to destroy a city — even within Russia's borders — if that's what it takes to achieve his political goals.
As Russian prime minister in 1999, Putin launched a brutal military campaign against Chechnya to keep that province under Moscow's control. Russian troops largely destroyed its capital Grozny in the process. Thousands of civilians were killed there.
"I think Putin is going to have a hard time walking this back," Jacobs said. "He's going to keep going until he gets it done ... at some enormous cost. To Ukraine, to Russia, and perhaps even to allies."
Other major cities in Ukraine are likely already surrounded by Russian forces, according to a U.K. Ministry of Defence update released Tuesday.
"Ukrainian forces continue to hold the cities of Kharkiv, Kherson and Mariupol however all three cities are now likely encircled by Russian forces," the ministry said.
Ukraine's government said on Tuesday that Kharkiv, its second-biggest city, was under intensifying shelling from Russian forces, NBC News reported. That city is home to around 1.5 million people.
At least 10 people were killed and 35 wounded on Tuesday by Russian rockets in the center of Kharkiv, Interior Ministry adviser Anton Herashchenko said in a post on social media.
Stopping the convoy
Kyiv is not yet surrounded, but a huge, 40-mile Russian column is proceeding slowly from the north toward the city to link up with forces to the south and encircle the capital.
Ukrainian troops have put up fierce resistance against Russian invaders north of Kyiv, but without sufficient air power, they have not been able to force the convoy into retreat.
The ongoing fight around the convoy is critical for the Ukrainians and Kyiv. It's important, Jacobs said, that more Western weapons get to the Ukrainians before they're cut off.
"If the Ukrainians can ... bring enough weapons to bear on that convoy, they're liable to wind up stopping it long enough so that the Ukrainians can get resupplied by the West," he said.