the nascent Ukrainian operation to push occupying Russian troops out of the south of the country will be costly for both sides, an American veteran training Ukrainian forces has told Newsweek.

Erik—who did not wish to share his full name for security reasons—is a 26-year veteran of U.S. special forces. Erik is now volunteering as a member of the Mozart Group, founded by former Marine Corps Colonel Andy Milburn, which has been training Ukrainian troops and engaging in humanitarian missions across the war torn nation.

Erik and his colleagues have been preparing Ukrainian forces now taking part in a counter-offensive towards the occupied city of Kherson and its eponymous surrounding region.

The operation—details of which are being closely guarded by the Ukrainian state—will require a change in mentality and resilience against high casualties, Erik said.

NEWSWEEK NEWSLETTER SIGN-UP >

"Ukraine, in many ways, has been on the defensive," the American veteran told Newsweek from close to the southern front line on Friday. "When you're on defense the circumstances for victory are better on your side than if you're on offense."

"They're having to change the mentality," he said.

Each phase of fighting in Ukraine has been shaped by local conditions. Russia's early drive towards Kyiv became bogged down in the thick forests and wetlands to the north of the capital. In the east and south, Ukrainian and Russian troops have been fighting over flat, open steppes dotted with settlements.

NEWSWEEK SUBSCRIPTION OFFERS >

Retaking Kherson and other occupied southern cities like Melitopol will require advancing Ukrainian troops to root the Russians out of urban areas.

"Urban combat is very bloody," Erik said. "There's a lot of casualties, it doesn't matter how well trained you are."

"The key is going to be learning how to operate in those areas," he continued. "There are so many considerations when you're doing urban operations that are different: communications, supply, logistics, casualties. Everything's different in urban areas; it's triply complicated."

"I've been in a lot of wars. And no matter how well prepared your people are and your troops are, nobody's ever prepared for the massive number of casualties. It's just the way it is. That's why Russia has made it a state secret when soldiers die."