Six more members of a Japanese death cult responsible for a sarin gas attack on the Tokyo subway that killed 13 people have been executed.

Thirteen members of the group have now received the death penalty for a series of crimes including the attack, the deadliest in Japan since the end of the Second World War.

The cult's leader Shoko Asahara was one of seven hanged three weeks ago, before a further six were executed on Thursday.

Japan has never carried out death sentences on so many people in a single month, justice minister Yoko Kamikawa said.

She called their crimes “unprecedentedly heinous” and said they should never be repeated.

The cult, named Aum Shinrikyo, or Supreme Truth, foresaw an apocalyptic showdown and had amassed a huge arsenal of both biological and conventional weapons during the 1990s.

It has been found responsible for a number of murders and various other crimes, including a deadly attack using the nerve agent VX in 1994.

However, the group’s most infamous attack remains the 1995 sarin gas release on the Tokyo subway, which left 13 people dead and as many as 6,000 others injured.

Carried out in the district which houses Japan’s parliament, Aum Shinrikyo members pierced plastic bags full of the toxic agent in a bid to hinder police investigations into the cult and possibly trigger the apocalypse they believed was approaching.

Four of the six executed on Thursday, Masato Yokoyama, Yasuo Hayashi, Toru Toyoda and Kenichi Hirose, released sarin on the subway.

Two others, Kazuaki Okasaki and Satoru Hashimoto, were convicted of separate crimes – the 1989 murders of an anti-Aum lawyer, his wife and 1-year-old baby and a 1994 sarin attack in Matsumoto, which killed seven people and injured more than 140.

Japan had not executed more than 10 people in a year since 2008, according to Amnesty International

The human rights organisation said four of those hanged on Thursday had requests for a retrial pending.

“This unprecedented execution spree, which has seen 13 people killed in a matter of weeks, does not leave Japanese society any safer,” Hiroka Shoji, East Asia researcher at Amnesty International, said.

“The hangings fail to address why people were drawn to a charismatic guru with dangerous ideas.

“The taking of a life in retribution is never the answer. It is high time for the Japanese authorities to establish an immediate moratorium on all executions and promote an informed debate on the death penalty as first steps towards its abolition.”

The executions were announced only after they had happened, as is the practice in Japan.

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Asahara, whose original name was Chizuo Matsumoto, founded Aum Shinrikyo in 1984.

Bearded and partially blind, the self-proclaimed guru recruited scientists and others to his cult, attracting people who were disillusioned with a modern, materialistic lifestyle.

During an eight-year trial, he talked incoherently, occasionally babbling in broken English, and never acknowledged his responsibility or offered meaningful explanations.

The cult once claimed 10,000 members in Japan and 30,000 in Russia. It has disbanded, though nearly 2,000 people continue to follow its rituals in three splinter groups, monitored by authorities.