Nina Schick, the author of "Deepfakes," said the video looked like "an absolutely terrible faceswap," referring to programmes that can digitally graft one person's face onto another's body -- part of a wider family of computer techniques that can create hyper-realistic forgeries known as "deepfakes."

Television station Ukraine24 said in a Facebook post that the video was broadcast by "enemy hackers" and was "FAKE! FAKE!" The station could not immediately be reached for further detail and Ukraine's cyber watchdog agency did not immediately return messages seeking comment. But Ukraine's Ministry of Defence later released a video from the real Zelensky apparently dismissing the footage as a "childish provocation."

"We are not going to lay down any weapons until our victory," he said.

Ukrainian officials have been warning of the danger of deep fakes, especially after Moscow's forces were denied a quick victory on the battlefield following their Feb. 24 invasion.

Two weeks ago, Ukraine's military intelligence agency put out a short video alerting the country to the danger of deep fakes, alleging that the Kremlin was preparing a stunt involving one.

The Russian embassy in Washington did not immediately return a message seeking comment.

Schick called the fake Zelensky video "very crude," but warned that it was a matter of time before the technology became more accessible.

"Expect fakes like this to become easier to produce while appearing highly authentic," she said.