US officials have said they had no choice but to launch a special forces rescue operation in Yemen that ended with Al-Qaeda killing an American photojournalist and a South African teacher.
The hostages -- 33-year-old American Luke Somers and 57-year-old South African Pierre Korkie -- were killed by their captors when US commandos stormed an Al-Qaeda hideout early on Saturday.
The failed raid came after the kidnappers had threatened in a video to kill Somers within 72 hours, and just a day before Korkie was due to be released under a negotiated deal.
Calling the murders "barbaric," President Barack Obama said he had authorised the rescue attempt because the video and other information "indicated that Luke's life was in imminent danger".
"The United States will spare no effort to use all of its military, intelligence, and diplomatic capabilities to bring Americans home safely, wherever they are located," he said.
"There were compelling reasons to believe Somers's life was in imminent danger," US Defence Secretary Chuck Hagel said in a statement after the operation.
"It was either act now and take the risk, or let that deadline pass. And no one was willing to do that," said a senior defence official who was with Hagel in the Afghan capital Kabul.
British-born Somers, 33, had worked as a freelance photographer for the BBC and spent time at local newspapers, including the Yemen Times, before being abducted in Sanaa in September 2013.
Korkie and his wife Yolande, who had worked as teachers in Yemen for four years, were kidnapped by Al-Qaeda in the Arabian Peninsula (AQAP), the group's powerful Yemeni branch, in May 2013.
She was released in January and the Gift of Givers charity said logistical arrangements had already been put in place to fly Korkie out of Yemen on Sunday.
Saturday's operation saw 40 US commandos dropped by helicopter 10 kilometres (six miles) from where Somers and Korkie were being held in the southeastern province of Shabwa, officials said.
They made their way to the Al-Qaeda hideout by foot, but were discovered about 100 metres (yards) away.
A short but intense firefight -- lasting five to 10 minutes -- erupted, the US defence official said.
"When the element of surprise was lost, and a firefight ensued, we believe that is when (the hostages) were shot," the official said.
One hostage -- it was not clear who -- died en route to the USS Makin Island off Yemen, and the other died on the operating table, officials said.
The defence department said in a statement that the operation had been conducted with the "assistance and cooperation" of Yemen's government and security forces. It said several of the AQAP militants were killed.
Yemeni authorities said 10 militants were killed in the mission and that four members of its security forces were wounded.
Local forces were on alert in two southeastern provinces that are strongholds of Al-Qaeda, the interior ministry said.
Friends and family described Somers as a committed journalist who sought to document the lives of ordinary people amid turmoil.
Yemen has been wracked by unrest since a 2011 uprising forced president Ali Abdullah Saleh to step down, with rival militias battling for control of parts of the impoverished country.
Korkie's family had been expecting him home soon, said the Gift of Givers charity, which had been negotiating his release.
"The psychological and emotional devastation to Yolande and her family will be compounded by the knowledge that Pierre was to be released by Al-Qaeda tomorrow," it said on Saturday.
"Three days ago we told her 'Pierre will be home for Christmas'."
A US State Department official said Washington was aware two hostages were at the location but did not know the identity of the second.
The threat to kill Somers followed the murders of five Western hostages since August by the Islamic State jihadist group which controls parts of Syria and Iraq.
Experts said the deaths during the raid were likely the result of intelligence failures.
Mustafa Alani, a security analyst from the Geneva-based Gulf Research Centre, said there appeared to have been a lack of information "on the area where the hostages were being held and the movements of the kidnappers".
"The Americans have the capacity to put commandos into action but their intelligence remains weak" in Yemen, he said.
AQAP, considered one of the group's most dangerous affiliates, launches frequent attacks in Yemen and has also organised a series of attempted bomb attacks on Western targets.
It is believed to be holding several other foreign hostages, including a Briton and Saudi and Iranian diplomats.