"The callous disregard for Luke's life is more proof of the depths of AQAP's depravity, and further reason why the world must never cease in seeking to defeat their evil ideology," U.S. President Barack Obama said in a statement.

He said he had authorized the attempted rescue and said the United States would "spare no effort to use all of its military, intelligence and diplomatic capabilities to bring Americans home safely, wherever they are located".

Gift of the Givers said in a statement on its website: "We received with sadness the news that Pierre was killed in an attempt by American Special Forces, in the early hours of this morning, to free hostages in Yemen."

It added: "The psychological and emotional devastation to (Korkie's wife) Yolande and her family will be compounded by the knowledge that Pierre was to be released by al Qaeda tomorrow ... Three days ago we told her 'Pierre will be home for Christmas'."

A South African government spokesman declined to comment.

There was no new information about three other hostages, a Briton, a Turk and a Yemeni, who had previously been held alongside Somers and Korkie, a Yemeni security official told Reuters.

Lucy Somers, the photojournalist’s sister, told the Associated Press that she and her father learned of her brother's death from FBI agents at 0500 GMT (12 a.m. EST) Saturday.

"We ask that all of Luke's family members be allowed to mourn in peace," she said from London.

IMMEDIATE DANGER

Kerry and U.S. Secretary of Defense Chuck Hagel both said the decision to mount the raid was based on fears that AQAP planned to kill Somers.

"Earlier this week, AQAP released a video announcing that Luke would be murdered within 72 hours. Along with other information, there was a compelling indication that Luke's life was in immediate danger," Kerry said.

Hagel, in Kabul, said: "There were compelling reasons to believe Mr Somers' life was in immediate danger." He said Somers and another hostage, who he did not name, had been killed by al Qaeda militants.

U.S. officials on Thursday said American forces had already attempted to rescue Somers, without giving details. Yemeni officials had previously disclosed the release of six Yemenis, a Saudi and an Ethiopian hostage in a raid on Nov. 25.

The operation involved an air strike followed by a raid by U.S. and Yemeni forces, a local security official said. It took place in Dafaar village in Shabwa Province and targeted an al Qaeda group headed by Mubarak al-Harad.

"It's a very small village with only 20-40 houses. There were very quick clashes with the gunmen and then it was all finished," a tribal source from the area told Reuters.

A senior Yemeni intelligence official said: "When the forces entered the place where the hostages were being held, they called on the kidnappers to give themselves up because they were surrounded on all sides.

"But the kidnappers immediately killed two hostages, which prompted the forces to open fire on the kidnappers. They tried to give first aid to the hostages but they had lost their lives."

AQAP on Thursday released a video showing a man it said was Somers saying: "I'm looking for any help that can get me out of this situation. I'm certain that my life is in danger". Reuters was not able to independently verify the authenticity of that video, which was reported on by SITE Monitoring.

Reuters