Japan has carried out another round of "secret" executions, bringing to eight the number of inmates sent to the gallows under the year-old administration of Shinzo Abe.

Media reports said two men had been hanged in the fourth round of executions since Abe took office last December. Previous hangings took place in February, April and September, suggesting that the government plans to carry them out every few months.

On Thursday, Mitsuo Fujishima, 55, was hanged for two murders in 1986, while Ryoji Kagayama, 63, had been convicted of killing two people in 2000 and 2008, media reports said.

Japan has brushed aside calls by Amnesty and the European Union to abolish the death penalty, citing strong public support for the punishment.

Opinion polls regularly put support for capital punishment at over 80%.

Thursday's hangings came just after the parliamentary recess began, and as Abe's approval ratings began to tumble following the passage last week of a controversial secrecy law.

The justice minister, Sadakazu Tanigaki, who signed the execution orders, said the two executed men had been guilty of brutal crimes.

"The executions were carried out after careful consideration of their cases," Tanigaki told reporters, adding that he saw no need to respond to calls to review the opaque way Japan administers capital punishment.

Prisoners, who spend years, even decades, on death row, typically are not told of their execution until hours before they are led to the gallows. Their lawyers and relatives are informed only after the execution has been carried out.

In a report published in 2008, Amnesty said inmates in Japan were being driven insane and exposed to "cruel, inhuman and degrading" punishment.

The executions in February this year were the first since September 2012.

The previous government, led by the centre-left Democratic party of Japan, executed nine people during its three years and three months in office. That included an 18-month period in which no one was hanged. The resumption of executions in March 2012 angered campaigners, who believed Japan was moving toward abolition.

On Thursday, Amnesty said Japan was increasingly out of step with the international community.

"The fast pace at which the Abe administration is conducting executions goes directly against the international community's repeated calls to abolish capital punishment," the group's Japan branch said in a statement.

Japan now has 129 inmates on death row, including Shoko Asahara, leader of the doomsday cult behind the 1995 sarin gas attacks on the Tokyo subway in which 13 people died and thousands were injured.

Japan and the US are the only G7 countries to retain capital punishment, along with more than 50 other countries, including China and Iran. More than two-thirds of countries, including all EU member states, have ended executions in law or practice.