TOKYO (AFP) - The mother of a second Japanese hostage apparently beheaded by the Islamic State in Iraq and Syria said on Sunday that she "can't find the words" to describe the death of her son, journalist Kenji Goto.

"It's deplorable, but Kenji is gone," Mr Goto's sobbing mother Junko Ishido told reporters at her home in a Tokyo suburb.

"I can't find the words to describe how I feel about my son's very sad death."

Mr Goto's brother Junichi Goto said he had been holding out hope, "but that's not possible anymore," he was quoted as saying by public broadcaster NHK.

Earlier this week, Mr Goto's wife Rinko broke her silence to plead for her husband's return.

"My husband is a good and honest man who went to Syria to show the plight of those who suffer," she said.

"I beg the Jordanian and Japanese governments to understand that the fates of both men are in their hands," she said, referring to the Jordanian pilot also held captive by the militants and threatened with death.

The couple's second child was born several weeks before Mr Goto left for Syria.

Born in Sendai, Miyagi, in 1967, Mr Goto was a freelance journalist who established a video production company, Independent Press, in Tokyo in 1996, supplying documentaries on the Middle East and other regions to Japanese television networks, including public broadcaster NHK.

Mr Goto had apparently been out of contact since late October after telling family that he intended to return to Japan, NHK reported.

In early November, his wife received e-mail demands for about 1 billion yen (S$11.5 million) in ransom from a person claiming to be a member of ISIS, Fuji TV said.

The e-mailed threats were later confirmed to have come from a sender implicated in the killing of US journalist James Foley, Fuji TV said.

In video footage he filmed around the time he entered Syria, Mr Goto holds identification papers and his Japanese passport and explains that he is aware of the risks.

"Whatever happens, I am the one who is responsible," he says. "I am asking you, Japanese people, do not place responsibility on the people of Syria. Please. I am sure I will come back alive, though."

Mr Goto's mother later pleaded with the hostage takers to release her son, whom she described as a humanitarian working to better the lives of children.

"I say to you people of the Islamic State, Kenji is not your enemy," a sobbing Ms Junko Ishida told foreign journalists.

"Please release him... Kenji was always saying 'I hope to save lives of children on battlefields'. He was reporting war from a neutral position."