A bold offensive from Ukraine's military has seen the nation reclaim major areas from Russian forces – and now, the invaders seem to be in panic mode.

Ukraine recently went on a counteroffensive across the eastern region of Kharkiv, which had been occupied by Russia.

Earlier this month, three dozen towns and villages were liberated as Russian forces simply fled with little resistance.

Nikolay Mitrokhin, a Russian expert at Germany's University of Bremen, told Al Jazeera that Ukraine's success had stunned experts.

"Within four days, Ukraine nullified four months of success of the Russian army that cost them a huge amount of victims," he said.

Now, that incredible win seems to have spooked Russia, with Denis Pushilin, the leader of the Russian-backed administration in Donetsk, pushing for immediate referendums on the area officially joining Russia.

Ukrainian President Volodymyr Zelenskyy said in a televised address to his people that the urgent request proved the counteroffensive had scared the invaders.

"The occupiers are clearly in a panic," Zelenskyy said, noting that "speed" was now the top priority in freed regions.

"The speed at which our troops are moving. The speed in restoring normal life," he said, suggesting he would also be calling on the global community to assist with supplying more weapons and aid.

On Sunday, Zelenskyy also said the military was preparing "for the liberation of more cities" soon.

"Perhaps it seems to someone now that after a series of victories we have a certain lull. But this is not a lull. This is preparation for the next sequence," Zelenskyy said.

"Izium, Balakliya, Kupyansk and the Kharkiv region in general are the cities and communities that we have liberated. These words are heard now. They are heard everywhere."

He added that liberating Donetsk, Horlivka and Luhansk were next on the agenda.

However, Serhiy Gaidai, the head of the Luhansk regional military administration, has confirmed that several areas within the region had already been liberated, including the town of Kreminna and the village of Bilohorivka.

Ukrainian President Volodymyr Zelenskyy sings Ukrainian national anthem during his visit in Izium, Kharkiv region, Ukraine. Photo / AP

He said that Ukrainian forces were now preparing to retake the entire Luhansk region.

"There will be a hard fight for every centimetre of Luhansk land," Haidai said on Telegram.

"De-occupation is not far away."

It comes as the US Air Force commander in Europe, General James Hecker, confirmed that Ukraine had managed to shoot down around 55 Russian fighter jets since the beginning of the war, and that Russia had never managed to gain the upper hand in the air.

According to the British Ministry of Defence, four of those jets were "highly likely" to have been lost within the last 10 days.

Meanwhile, the bodies of two children have been exhumed from a mass grave in the city of Izyum, which was discovered in a forest earlier this month.

Investigations are ongoing, but 450 bodies believed to be of "tortured" people, including many civilians and "entire families", have already been discovered.

Kharkiv Governor Oleh Synehubov said bodies had been found with hands tied behind their backs, with most believed to be of civilians who carried "signs of violent death".

However, Russia has denied being behind the atrocities.

"It's a lie, and of course we will defend the truth in this story," Kremlin spokesman Dmitry Peskov said.