WASHINGTON — The US manufacturer of the Taser stun gun has advised police not to aim the weapons at the chests of suspects after admitting heart risk concerns for the first time.

Taser International stressed that suffering an "adverse cardiac event" after being zapped was "extremely unlikely," but human rights groups say hundreds of people have been killed by the electroshock weapons.

In a bulletin dated October 12, the Arizona-based company issued new guidelines saying it had "lowered the recommended point of aim from center of mass to lower-center of mass for front shots."

"When possible, avoiding chest shots with ECDs (Electronic Control Devices) avoids the controversy about whether ECDs do or do not affect the human heart," it explained.

"Researchers have concluded that a close distance between the ECD dart and the heart is the primary factor in determining whether an ECD will affect the heart. The risk is judged to be extremely low in field use," it said.

Tasers, which pack a 50,000-volt punch that can paralyze targets from up to 10 meters (30 feet) away, are used by several police forces around the world, including in Britain, Canada, France, Greece, Israel, and the United States.

Human rights activists have long criticized the stun guns, challenging previous claims from the manufacturer that they are a safe, non-lethal alternative to handguns.

More than 350 people died between 2001 and December last year after being stunned by the weapons, according to Amnesty International, which has been monitoring Taser-related deaths.