Just days after Japan was praised by Amnesty International for suspending death sentences, it executed three death row inmates by hanging, ending its 18-month moratorium on capital punishment.

But with the recent executions, Amnesty has warned that there are 130 prisoners in Japan still awaiting the final verdict in their cases. Several suffer from mental illness, says the human rights group.

Amnesty noted an increase in the number of executions worldwide in 2011. It directed its criticism largely toward the United States, the only G8 country to carry out executions last year. Japan, China, Iran and Iraq are also among the 58 countries throughout the world that still retain capital punishment.

Amnesty highlighted the treatment of prisoners with mental illness. The US has refused to stop imposing the death penalty on mentally ill prisoners, saying it supports the recommendation only for those with certain intellectual infirmities.

Amnesty was shaken by the cases of Reginald Brooks, 66, who was executed in Ohio in 2011 despite suffering from paranoid schizophrenia, and Eddie Duval Powell, a 41-year-old African-American from Alabama, who was also executed in 2011 despite having a learning disability.

China remains responsible for the majority of the world's executions, although reliable figures are not available since the use of the death penalty is considered a state secret. Beijing claims it has reduced its use of capital punishment since 2007 but the claims cannot be independently verified.

Belarus carried out two executions in 2011 - the only country in Europe to do so.

Amnesty recorded 360 acknowledged executions in Iran in 2011 but suspects many more may have been carried out. The death penalty is used there for charges ranging from sodomy and corruption to adultery and apostasy.

Iraq also rarely discloses information about its use of capital punishment, though Amnesty said at least 68 people were executed in 2011, including two foreigners and three women. According to the organisation, 735 death sentences were referred to Iraqi President Jalal Talabni for ratification between January 2009 and September 2011, of which 81 were ratified.