Kenji Goto's mother Junko Ishido has spoken of her grief after Islamic State released a video apparently showing the murder of her son.
Speaking to reporters after the video was released Ms Ishido broke down several times.
"I am too upset to find the words to express myself. My son's last act was to go to Syria to help a fellow Japanese. So I want people to understand my son's kindness and courage."
Mr Goto's brother, Junichi, also paid tribute to the journalist. He said: "I had hoped to give thanks for his return alive. But, as his brother, this outcome is very regrettable."
The African Union (AU) has authorised a force of 7,500 troops from Nigeria, Chad, Cameroon, Niger and Benin to fight Boko Haram militants, Ghanian President John Mahama told journalists on Saturday.
He added that the nations will meet in the Cameroon capital next week to decide on the joint operation's command structure.
Mr Mahama said: "When they meet in Yaounde the rules of engagement will be agreed by the nations constituting the force. It will allow them to move across borders because Boko Haram does not recognise borders."
US President Barack Obama has condemned the 'heinous murder' by Islamic State of Japanese hostage Kenji Goto, the White House said.
In a statement Mr Obama said: "The United States condemns the heinous murder of Japanese citizen and journalist Kenji Goto by the terrorist group ISIL.
"Standing together with a broad coalition of allies and partners, the United States will continue taking decisive action to degrade and ultimately destroy ISIL."
Chadian forces have killed 120 militants from Boko Haram in a battle in the north of neighbouring Cameroon that began when the insurgents attacked its troops, according to a statement from the army.
Three Chadian soldiers were also killed during the fighting.
Boko Haram has recently launched cross-border attacks from Nigeria into Cameroon and Chad as part of its drive for an Islamist state in the northeast of Nigeria.
Chad and Cameroon have stepped up troop deployments to fight the militants.
The Prime Minister has issued a statement following reports of the murder of Japanese journalist Kenji Goto.
I utterly condemn what appears to be the despicable and appalling murder of Kenji Goto. It is a further reminder that ISIL is the embodiment of evil, with no regard for human life.
My thoughts and prayers are with the families of Mr Goto and Mr Yukawa and the Japanese people as they come to terms with the murder of two innocent citizens in such a brutal manner. Britain stands united with Japan at this tragic time and we will do all we can to hunt down these murderers and bring them to justice, however long it takes.
I welcome Prime Minister Abe's steadfast commitment to continue Japan’s active role, working with international partners, to secure peace, stability and prosperity in the Middle East. The humanitarian aid they are providing in the region is a vital part of helping the local communities that are being persecuted by the same ISIL terrorists who murdered our innocent men.
The Japanese Government is right not to bow to terrorism. The way we will defeat ISIL is not by giving in to these terrorists but by confronting them and their poisonous ideology. With determination and patience, we will work together with Japan and our other allies to extinguish this terrorist threat and to stand up for the values of tolerance and peace."
Japan's defence minister Gen Nakatani has said a purported Islamic State video that seems to show the murder of hostage Kenji Goto "appears genuine".
Japan's prime minister Shinzo Abe has condemned the apparent killing of captured journalist Kenji Goto by Islamic State fighters.
Speaking this evening, before the video was verified, he said he would never forgive these terrorists.
Goto has been held by Islamic State militants since October. Martha Fairlie reports.
Japan's prime minister Shinzo Abe has condemned the apparent killing of Kenji Goto and vowed that his country will not give into terrorism.
I feel strong indignation at this inhumane and contemptible act of terrorism. I will never forgive these terrorists.
Japan will work with the international community to bring those responsible for this crime to justice.
A spokesman for the Japanese government has condemned a video released by Islamic State extremists which appears to show journalist hostage Kenji Goto being beheaded.
Japanese chief cabinet secretary Yoshihide Suga said the cabinet would be meeting to discuss its response.
I cannot help feeling strong indignation that an inhuman and despicable act of terrorism like this has been committed again.
We resolutely condemn this.
The US government has said it "strongly condemns" actions by Islamic State militants after a video was posted online appearing to show the execution of Japanese hostage Kenji Goto.
In a statement, the White House called for all remaining hostages to be released and said it was working to confirm the authenticity of the video.
We have seen the video purporting to show that Japanese citizen Kenji Goto has been murdered by the terrorist group ISIL.
We are working to confirm its authenticity. The United States strongly condemns ISIL's actions and we call for the immediate release of all remaining hostages. We stand in solidarity with our ally Japan.