Kyiv, Ukraine - Ukrainian President Volodymyr Zelensky demanded that the world respond to a video posted online that allegedly shows a Ukrainian prisoner of war being beheaded.

"This is a video of Russia as it is," Zelensky said on Wednesday. He added it was neither an accident nor an isolated incident.

No one would understand if state leaders did not react to the video, the president insisted. "Action must be taken now!"

Meanwhile, Ukrainians must focus on the front and drive the occupiers out of the country. "Smashing the occupier, sentences for the murderers and a tribunal for the state of evil," are the main tasks now, Zelensky said.

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His comments came after the grisly video started circulating online on Tuesday. The trees in the footage are green, suggesting the video was recorded last year.

The Ukrainian secret service is investigating the video. "We will find these brutes. If necessary, we will find them wherever they are: underground or from beyond the grave," said Vasyl Malyuk, chief of the Security Service of Ukraine (SBU).

In response to the video, Kyiv's Foreign Minister Dmytro Kuleba called Russia "worse" than the Islamic State terror group and demanded Russia's expulsion from the United Nations.

Meanwhile in Moscow, the Kremlin cast doubt on the authenticity of the video.

"First of all, we live in a world of fakes and therefore we have to check the authenticity of the footage," Kremlin spokesman Dmitry Peskov said, according to the Interfax news agency.

These are "horrific images," Peskov admitted, but said it had to be established whether the beheading actually took place.

Then, he said, the authorities should examine which side committed the crime.