They are the country's first executions under Japan's new Prime Minister, Shinzo Abe.

Japan, like the United States, is one of the world's few developed countries to exercise capital punishment.

It has been reported that those hanged included a taxi driver convicted of four murders. The last hanging in Japan was in September 2005.

The inmates have been identified as Yoshimitsu Akiyama, Yoshio Fujinami, Michio Fukuoka and Hiroaki Hidaka.

Kyodo News and Jiji Press report that Hidaka, a taxi driver in Hiroshima, was convicted of murdering a girl and three women.

Akiyama, 77, was said to be the oldest and had spent the longest time of the four on death row, having been sentenced in 1987, Kyodo added.

International criticism

Under former Justice Minister Seiken Sugiura, no execution orders were signed during his 11 months in office, because he said they went against his Buddhist beliefs.

But he was replaced by Jinen Nagase three months ago, when Mr Abe took office.

Japan has faced international criticism for giving inmates little notice that they will be executed, so as to prevent last-minute appeals.

Amnesty International's Japan office has issued a statement condemning the executions.

Critics have also noted that Japan conducts executions while parliament is in recess, allegedly to avoid debate on the topic.

The last session of parliament ended on Tuesday.