United Nations experts had a damning assessment of taser use in U.S. jails, saying footage they viewed revealed the need for investigations and in some cases showed abusive use of the devices so severe it amounted to torture.

The watchdogs viewed video footage obtained by Reuters of 22 incidents in four jails: Franklin County, Ohio; Cheatham County, Tennessee; Franklin County, Arkansas; and McCurtain County, Oklahoma.

In addition to showing the need for probe, Nils Melzer, the U.N. special rapporteur on torture, said most of the incidents "are likely to merit prosecution."

Reuters' multi-part investigation on Tasers includes chilling details of their use on people detained—people who were often posing no threat, and may have been pinned down or handcuffed.

"When you use a tool like this on an incapacitated person, to me it certainly amounts to cruel and degrading treatment," Melzer told the news agency.

One case Melzer said "looks like a case of torture" was the 2009 tasing of 20-year-old pregnant and nearly naked Martini Smith. After having been in handcuffs for six hours, Smith told the Franklin Co., Ohio officers she was unable to comply with their demand to take her tongue ring out. The officers tased her in the chest. She miscarried five days later.

"It's not that she physically resisted or would have been dangerous to the officer," Melzer said.

Another case of torture Melzer identified in the video footage was a 2016 incident in which Cheatham County officers repeatedly tased a 19-year-old, despite his being strapped in a restraint chair.

Jens Modvig, chairman of the U.N. Committee against Torture, which monitors states' compliance with the global treaty against torture, also condemned the incidents in the videos. He said they revealed "blatant abuse."