US hostage Luke Somers and SA Pierre Korkie killed during Yemen rescue bid
UK-born US journalist Luke Somers and South African teacher Pierre Korkie have been killed by al-Qaeda militants in Yemen during a failed rescue bid.
Saturday's operation was carried out by joint US and Yemeni special forces in the southern Shabwa region.
US President Barack Obama condemned the "barbaric murder" of both hostages.
They were being held by militants from al-Qaeda in the Arabian Peninsula (AQAP), regarded by the US as one of the deadliest offshoots of al-Qaeda.
The group is based in eastern Yemen and has built up support amid the unrest which has beset the impoverished country since the overthrow of President Ali Abdullah Saleh in 2011.
Analysis: Frank Gardner, BBC Security correspondent
This is America's third failed hostage rescue mission in the Middle East in less than six months.
In July its Special Operations commandos launched a raid on an Islamic State hideout near Raqqa in Syria in a bid to free James Foley and other US and British hostages. They were too late: the men had been moved just days earlier.
The same thing happened in November with Luke Somers in Yemen. US operatives found the cave in a remote province where al-Qaeda had been hiding him and other hostages. They freed a number of captives but once again, the westerners had been moved only days before.
In this latest raid in Yemen their intelligence was accurate. They correctly located where Luke Somers was being held. But once his captors were aware they were being attacked they killed both him and Pierre Korkie proving that hostage rescues are notoriously risky and have only a limited chance of success.
President Obama said he authorised the raid to rescue Mr Somers and other hostages held in the same location.
He said information had "indicated that Luke's life was in imminent danger".
Both Mr Somers and Mr Korkie were "murdered [by] terrorists during the rescue operation", Mr Obama said.
He offered his "thoughts and prayers" to Mr Korkie's family, saying: "Their despair and sorrow at this time are beyond words".
A charity working with Mr Korkie said that they had expected his release on Sunday, and that the US rescue attempt had "destroyed everything".
A number of militants were also killed in the operation.
Senior US officials, speaking on condition of anonymity, told AP news agency that they believed both men were shot by their captors as the raid unfolded, and eventually died of their injuries.
Both hostages were alive when US forces rescued them, pulling them onto a plane that flew to a nearby US naval ship, officials said.
However, Mr Korkie is believed to have died during the flight, while Mr Somers died on the USS Makin Island, they added.
A spokesman for South Africa's Department of International Relations told local media: "We were informed this morning that Mr Pierre Korkie was killed in an operation by American forces to rescue hostages, that he was caught in the crossfire."
US special forces were in possession of Mr Korkie's body, which would be repatriated to South Africa, spokesman Nelson Kgwete added.
'Very much loved'
Mr Somers was born in Britain and UK relatives have been mourning his death in Kent.
His step-mother, Penny Bearman, told the BBC that he was "very much loved" by people in Yemen.
"Luke was a peace-loving person who cared for the Yemeni people and the Yemeni struggle," she said, speaking from Deal. "It is a tragedy that his life should end in this way."
A friend of Mr Somers, who did not wish to be named, described him as "one of the kindest and most dedicated people to Yemen I have ever met".
"He didn't concern himself with the politics of it or the important people," she told the BBC.
Instead, he spent "every waking hour with the neighbours and with youth activists, just trying to get the voice of the everyday Yemeni person out to the world."
Meanwhile a charity working with Mr Korkie said that it was saddened by his death.
"Pierre Korkie was very sick - he had a hernia," Gift of the Givers' Yemen project director Anas Hamati told the BBC's Newshour.
Mediators had been working on an "arrangement to take him out", he said, adding: "His passport was ready, everything was ready.
"In that time the attack happened by US special forces in Yemen and that has destroyed everything."
Mr Korkie was abducted with his wife Yolande in May last year in Yemen's second city, Taiz.
She was freed on 10 January without ransom and returned to South Africa.
"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," the charity said in a statement.
Mr Somers, who was kidnapped in Yemen in 2013, appeared in a video this week appealing for help.
The footage showed a member of AQAP threatening to kill him unless unspecified demands were met.
Mr Somers worked as a journalist and photographer for local news organisations. His material appeared on international news outlets, including the BBC News website.
Luke Somers emails to the BBC
22 August 2013
With widespread attention very recently focusing on embassy closures and security threats, I strongly feel that such a glimpse as this photo essay provides can serve to balance such attention with the true-to-life interests of Yemen's citizens.
24 August 2013
Yemen isn't the most difficult place to live in. Always something to make you smile, you just sometimes have to step outside and find it. I actually feel pretty fortunate, as I'm the only foreigner I know of working directly and regularly in connection with the National Dialogue Conference. But soon enough, I need to depart and spend some time with my mom!
4 September 2013
I'm sure I will return to the Middle East - and with regards to Yemen, it's pretty much a must. It's an emptying thought, imaging rooting yourself so firmly in a place, only to never return. So return I should, return I must.
Mr Somers was kidnapped outside a supermarket in the Yemeni capital Sana'a in September 2013.
"Our thoughts are with his family at this difficult time," a BBC spokesperson said on Saturday.
UK Foreign Secretary Philip Hammond condemned AQAP for the killing of both hostages.
"Luke had close links with the UK and his family have spoken about Luke's life and his work, and that is how he should be remembered," he said.
He told the BBC the UK would not pay kidnap ransoms for its subjects.
"We know from our own intelligence that hostage takers go for hostages of nationalities where they believe ransoms will be paid. So paying ransoms makes your citizens more at risk of kidnap than not paying ransoms."
Another attempt to rescue Mr Somers last month had failed.