Taser restraint weapons, which incapacitate people with a 50,000-volt electric shock, still have not been fired by Thames Valley Police six months into a year-long trial.

However, the force has deployed Tasers 23 times in Oxfordshire to incidents requiring an armed response.

Supt George Wilson, head of tactical support, said: "Thames Valley Police has so far not fired a Taser, but the weapon has been deployed on several occasions.

"We have found that it is having a deterrent effect and helps us bring dangerous situations to a safe end."

Tasers are only used by police where conventional firearms could be drawn. They operate at a low current and are designed to be a less lethal alternative to guns.

However, earlier this year Supt Wilson admitted it could not be guaranteed that a Taser would not kill someone.

In May, 24 police, some with Tasers, surrounded a home in Aldbarton Drive, Headington, Oxford, after a 34-year-old man barricaded himself inside.

He allegedly threatened to shoot paramedics who called at his home amid fears he had overdosed. Officers eventually arrested the man.

A row has since broken out between Amnesty International and the Association of Chief Police Officers (ACPO) over how research into the effects of the weapon should be interpreted.

Amnesty International's UK director Kate Allen said: "Tasers can kill and have been linked to deaths in the US.

"Full medical trials must be conducted before the Home Office considers any wider deployment."

But ACPO has described Amnesty's comments as "unhelpful". It stated that a trial by British police should not be linked to human rights abuses abroad.

Spokesman Derek Talbot, Assistant Chief Constable of Northamptonshire, said using a Taser to overpower an offender meant officers would not have to stop them with a bullet.