An advisor to Ukrainian President Volodymyr Zelenskyy said during a briefing on Sunday that the next "week or two" could determine the outcome of the Russia-Ukraine War, according to a report.

Ukraine presidential advisor Oleksiy Arestovych described the importance of the ongoing struggle over the port city of Mariupol, saying its salvation or fall could determine the eventual outcome of the war, Ukrainian news agency Ukrinform reported.

Russia's lack of swift victory, international condemnation, and economic sanctions along with devastation and civilian losses across Ukraine have brought both countries to the negotiation table, but fighting — including a critical struggle for Mariupol — continues.

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The city could hold strong, or potentially fall, and the outcome of the war could then quickly follow, Arestovych said, according to Ukrinform.

"The next week or two will be uneven," he added. "There will be reports that we are winning, counterattacking and repelling in some areas. We are liberating our lands from the enemy."

RUSSIAN INVASION-FIRE-UKRAINE

Firefighters work at a site of a fire, as Russia's invasion of Ukraine continues, in Kharkiv, Ukraine March 16, 2022. REUTERS/Oleksandr Lapshyn (REUTERS/Oleksandr Lapshyn)

"However, there will also be information that in some parts of the front, particularly in the east, the situation is complicated. You need to be emotionally, organizationally and consciously ready for this," the adviser added.

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Should Ukrainian forces repel the Russian army from Eastern Ukraine and successfully defend Mariupol, it could have an advantage in negotiation efforts — or it could utilize the strategic city to ultimately emerge victorious in the war, the advisor speculated.

If Russia surrounds and is able to "finish" Mariupol, they would have a direct line from the Russian mainland to the Crimean Peninsula, in which Russian forces could be more quickly resupplied and reinforced, Arestovych said, according to the report.

A dog and Ukrainian army soldiers in the street on the outskirts of Kyiv.

A dog is seen in the middle of a street as Ukrainian army soldiers take part of a military sweep to search for possible remnants of Russian troops after their withdrawal from villages in the outskirts of Kyiv, Ukraine, Friday, April 1, 2022.  ((AP Photo/Rodrigo Abd))

"They understand this," the advisor said, speaking of both countries. "On the one hand, it gives us hope in the area of Kyiv, Chernihiv, Sumy region, Kharkiv region that we will get rid of the enemy there, we will repel it from there. Secondly, it means a possible, potential deterioration or a sharp deterioration of the situation around Mariupol and in the JFO area."

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After Russia's military troops invaded Ukraine nearly 40 days ago, the larger army's might has been repeatedly stalled by Ukraine's military determination and will to save their homeland, resulting in a slowed attack and severe losses for Russia.

Putin Zelensky

A photo illustration of Ukrainian President Volodymyr Zelenskyy (left) next to Russian President Vladimir Putin.  (UKRINFORM/ Ukrinform/Future Publishing via Getty Images  |  getty)

Across the country, in Northern Ukraine, the Ukrainian military is slowly retaking territories but troops have been forced to move cautiously through these areas as Russia's military left mines and other hostile devices before they vacated them, Ukrainian President Volodymyr Zelenskyy said in his nightly address.

The situation is "catastrophic," Zelenskyy said, as mines were discovered around civilian homes and abandoned equipment.

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Military members are also hesitant to clear human remains as "even the bodies of those killed" may have been booby-trapped by Russian forces, the president added.

The Associated Press contributed to this report.