Amnesty International warned Tuesday against a proliferation of Taser stun guns, saying they were responsible for dozens of deaths in the United States and should only be used in extreme cases.

In a report entitled "'Less than lethal?' The use of stun weapons in US law enforcement," the London-based human rights group urged governments to either limit their deployment to life-threatening situations or to suspend their use.

Industry claims that so-called "Conducted Energy Devices" are safe and non-lethal do not stand up to scrutiny, it said.

Amnesty said 334 people had died in the United States between 2001 and August 2008 after being zapped with Tasers, with medical examiners and coroners concluding that Taser shocks caused, or contributed to, at least 50 of the deaths.

"Tasers are not the 'non-lethal' weapons they are portrayed to be," said Angela Wright, the Amnesty researcher who wrote the report. "They can kill and should only be used as a last resort."

"The problem with Tasers is that they are inherently open to abuse, as they are easy to carry and easy to use and can inflict severe pain at the push of a button, without leaving substantial marks," she said.

"We are very concerned that electro-shock weapons such as Tasers have been rolled out for general use before rigorous, independent testing of their effects."

Darts fired by the Taser pack a 50,000-volt punch that can paralyse targets from 10 yards (metres) away.

Used in many countries, including Britain, Canada, France and the United States, Tasers are seen as a less dangerous alternative to using firearms.

Amnesty said a review of death cases suggested that Taser shocks "may exacerbate cardio-respiratory problems in individuals whose health is already compromised by drug abuse, exertion, heart disease, psychosis or positional restraint."

"Some of those who died had no underlying disease or drugs in their system, but collapsed after being subjected to repeated or prolonged shocks and/or shocks to the chest, heightening concern that these factors may increase a risk of death or injury, even in relatively healthy individuals."

"Amnesty International considers that enough information is already available to indicate that such devices are potentially lethal," it said, adding that Tasers had been used against pregnant women, schoolchildren and elderly people suffering from dementia.

At the end of November, Amnesty criticised the British government's decision to equip police in England and Wales with 10,000 Tasers, following earlier trials. Regular British officers do not carry guns.

Last Friday, prosecutors in Canada opted not to press charges against four police officers seen on video jolting a Polish immigrant with a Taser in Vancouver airport in October 2007. The 40-year-old man died within minutes.