The hangings would continue a pattern in which executions are carried out during holiday periods, particularly towards the end of the year.
The death penalty in Japan is carried out in secret without lawyers or family members being officially informed.
Even the condemned prisoners themselves are given only a few hours to prepare for their deaths.
"This move would fit a pattern from previous years in which executions were carried out during the parliamentary recess and holiday period to minimise public and parliamentary reactions to the use of the death penalty," Amnesty said last night.
"At a time when Japan should be taking a lead in protecting human rights in Asia, it continues to kill its own citizens," it said.
Six executions were carried out in Japan in 1998 and, in August this year, three convicted murderers were hanged after spending seven years each on death row in conditions of near total isolation.
Condemned prisoners are generally kept in solitary confinement, with tight restrictions on visits and even letters from the outside world.
The isolation takes its toll on the mental health of prisoners, some of whom are extremely elderly. Last month, a 15-year veteran of death row, Katsunori Ota, was found dead in Sapporo Detention Centre in the island of Hokkaido after slashing his neck with a razor while taking a bath.
Almost 150 prisoners have been sentenced to death in Japan, and 47 of those have exhausted all their opportunities for appeal.
Opinion polls generally show that most Japanese support capital punishment for murder, but the whole process of executing prisoners has been criticised by human rights groups for its arbitrariness and inconsistency.
Between 1989 and 1993 there were no executions in Japan, originally because the then justice minister, a devout Buddhist, refused to sign the necessary documents. In 1993 they resumed, and public support for the death penalty increased two years later with the nerve gas attack on the Tokyo subway by the religious cult, Aum Shinri Kyo, which killed 12 people and injured 6,000.
During the police investigations which followed, numerous other crimes came to light, including the murder of a human rights lawyer, his wife and baby, after he campaigned against the cult.
Outrage at the murder was particularly intense among lawyers, many of whom had previously been opponents of the death penalty.
There was a certain amount of debate in the Japanese media during 1997 with the hanging of Norio Nagayama, a 47-year old man who during 28 years in prison had become a popular author.
In December 1997, Amnesty International launched an appeal after receiving similar information about imminent executions, and there were no hangings that winter.
Warning of today's executions apparently came from lawyers of condemned prisoners who had been unofficially notified by authorities.
In the past, families are only officially informed after the hangings have been carried out.Reuse content