Washington DC - The US government is looking into a fake video that appeared on social media, showing State Department spokesperson Matthew Miller discussing Russia's war on Ukraine.

"I will say just as a warning to everyone, if you see a video of me at the podium and my tie color and shirt color change from the time I'm asked a question to the time I answer, it is probably a good indication that it's not real," Miller said in a press briefing on Wednesday.

The New York Times reported on the 50-second clip last week.

In the clearly manipulated but relatively authentic-looking video, Miller appears to suggest that the Russian city of Belgorod near the border with Ukraine is a legitimate target of Ukrainian attacks.

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Last week, Washington authorized Ukraine to use US-supplied weapons on a limited scale against targets on Russian territory to defend the north-eastern Ukrainian city of Kharkiv.

However, the comments made in the fabricated video go way beyond this and are largely false.

Miller said Washington was "continuing to assess to find the actual provenance of that video."

"We have seen the Russian government execute videos in the past and use this disinformation to fool their own people, as well as people of other countries. But we don't have an assessment of this official video," he added.

Miller said that it looked like videos of press conferences he conducted in the past were used as a base for the fake, insisting: "The audio is not me at all." It wasn't clear whether AI was used to put the clip together.

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"It is something that we are extremely concerned about. You have seen doctored videos of [President Joe Biden] in the past," Miller said. "It is something that we are working on as part of our broad efforts to combat disinformation."

Ukraine has sought permission to use weapons supplied by the West to strike Russian bases launching air attacks in the region. Several countries including the US and Germany have recently given the green light for their weapons to be used on a limited scale for that purpose.

Moscow has responded by warning of further escalation in the conflict, which has been running for over two years.