Japan has hanged a child killer and two other convicted murderers, its first executions since a conservative government swept to power in landslide elections in December.

Kaoru Kobayashi, 44, killed a seven-year-old girl and sent a photograph of the dead body to her mother in 2004, while Masahiro Kanagawa, 29, killed one person and injured seven others in a knife attack outside a suburban Tokyo shopping mall in 2008. He also murdered another man in a separate incident the same year.

The third man executed was Keiki Muto, 62, who strangled a bar owner for money in 2002.

"I ordered the executions after giving careful consideration to the matter," justice minister Sadakazu Tanigaki told a press briefing in Tokyo. "These were extremely cruel cases in which victims were deprived of their precious lives for very selfish reasons."

Kobayashi admitted that he abducted, sexually assaulted and murdered the seven-year-old, whose body was found in a gutter in western Japan.

The executions were Japan's first since two death-row inmates were hanged in September under a centre-left Democratic Party of Japan government.

'Cruel system'

The number of death-row inmates in Japan now stands at 134.

Japan did not execute any condemned inmates in 2011, the first full year in nearly two decades without an execution, amid muted debate on the rights and wrongs of a policy that enjoys wide public support.

But Tokyo resumed its use of capital punishment last March, with a government minister signing death warrants for three multiple murderers.

Apart from the US, Japan is the only major industrialised democracy to carry out the death penalty, a practice that has led to repeated protests from European governments and human rights groups.

International advocacy groups say the system is cruel because death row inmates can wait for their executions for many years in solitary confinement, and are only told of their impending death a few hours ahead of time.

On Thursday, Amnesty International's branch in Japan "strongly condemned" the latest executions.

"The Japanese government cannot be excused from abiding by international human rights standards by citing how the public are feeling," the group said in a statement.