EXCLUSIVE: Winning the war in Ukraine boils down to arms and timeliness, one top official told Fox News Digital following Kyiv’s major advances in Kharkiv this month.

"We need to keep the pressure on them," Yuriy Sak, adviser to Ukraine’s defense minister, told Fox News Digital. "If we allow them time to recuperate, if we allow them time to rebuild their capacity, then it will be more possible for them to strike back.

"We need to keep the momentum, and we need to keep going," he added, noting that the momentum must also continue to come in the way of international arms if Ukraine is going to be successful.

Ukrainian forces have taken back nearly the entire northern region of Kharkiv, liberating more than 400 towns and cities and forcing Russian troops to withdraw.

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The advances in Ukraine’s north were part of a long-awaited counteroffensive — first alluded to in mid-May — that also included an offensive push in Ukraine’s southern region.

Ukrainian soldier

Ukrainian forces are seen in Izyum after Russian forces withdrew from the city in Kharkiv Oblast, Ukraine, on Sept. 16, 2022. (Metin Aktas/Anadolu Agency via Getty Images)

But even Kyiv was surprised by its success in Kharkiv, where it managed to take Russian forces by surprise and force them to hastily retreat, in some cases back across Russia’s own borders.

"This was a mission that for a good reason was kept secret for a long time," Sak said. "The outcome of this counteroffensive actually exceeded even our own expectations."

The adviser said it was not only the amount of territory that exceeded Kyiv’s imagination but the success rate of the mission.

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Sak claimed that for every Ukrainian soldier killed in the counteroffensive, somewhere between "9 to 10" Russian soldiers were killed.

Reports have since surfaced suggesting there was a breakdown in command and control in Russia’s ranks as it withdrew from Kharkiv, prompting its soldiers to abandon not only military equipment but their own troops.

Sak said that Ukrainian defense officials had also received word that in an attempt to withdraw faster, Russian troops left behind soldiers killed in action and at times shot their own wounded, though he said these accounts still need to be confirmed.

Ukrainian military vehicles move on the road in the freed territory in the Kharkiv region,

Ukrainian troops retook a wide swath of territory from Russia on Monday, pushing all the way back to the northeastern border in some places, and claimed to have captured many Russian soldiers as part of a lightning advance that forced Moscow to make a hasty retreat. (AP Photo/Kostiantyn Liberov)

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"The Ukrainian army has had tremendous success," he said. "They know what they're fighting for. They want to win this war."

But Western officials have said it is too early to say if Ukraine’s ability to recapture some 3,300 square miles is a turning point or whether Russia has another operational ploy up its weathered sleeve.

"We're not fully aware of what they're capable of in this state of desperation — in which they definitely are in now," the adviser said. "We have to be prepared for everything. Again, this brings us back to the issue of haste and speed.

"They are learning from their own mistakes," he continued. "From a purely logical point of view, it is possible to assume that the next stages will be accompanied by a more sophisticated response."

Sak said Ukrainian defense officials are relying heavily on intelligence to determine their offensive strategy and would not guess at what Russia might do next.

"Our plan is simple: to keep fighting until we win this war," he added.

Ukraine soldiers izium

Ukrainian soldiers sit on infantry fighting vehicles as they advance near Izyum in eastern Ukraine on Sept. 16, 2022. (Juan Barreto/AFP via Getty Images)

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Sak pointed out that as Ukrainian forces were gaining in Kharkiv, Russia increasingly relied on hitting civilian targets and using "missile terror" in areas like the Zaporizhzhia nuclear power plant.

Ukraine continues to ask for greater air defense like anti-rocket systems, fourth-generation aircraft and longer-range missile systems. But the White House has not yet agreed to these big ticket items.

"We can be as successful as the amount of weapons that we receive," he said. "We understand that this war is far from over."