Two hostages, one of them an American missionary, have been killed in a rescue attempt by the Philippine army, the country's military chief says.

American Martin Burnham and Filipina nurse Ediborah Yap died in the operation, while Burnham's wife Gracia was wounded and is now in hospital, General Roy Cimatu said.

I know that every effort was made to rescue them safely, it's just unfortunate it ended this way

Doug Burnham, brother

The Christian missionary couple were kidnapped by Muslim separatist guerrillas in the southern Philippines in May last year.

Hundreds of troops, backed by American technology, launched the attack on Friday to try to rescue the couple as part of an extended rescue operation that has been going on for almost two weeks, officials said.

President Gloria Arroyo defended the military operation, saying "our soldiers tried their best to hold their fire for their (the hostages') safety. I salute our troops for their forbearance.

"The terror shall not be allowed to get away with this. We shall not stop until the Abu Sayyaf is finished. The battle shall go on, wherever it takes," she said.

US troops have been training local forces

Doug Burnham, brother of the missionary, said he was "grateful to everyone who has made an effort to try and rescue them".

"I know that every effort was made to rescue them safely, it's just unfortunate it ended this way."

US President George W Bush also expressed his grief over Mr Burnham's death.

"First let me say how sad we are that Martin Burnham lost his life," said Bush, adding: "I'm pleased that Mrs Burnham is alive. That's good."

He said he had been assured by President Arroyo that "justice would be done" to the terrorists who seized the American couple.

US troops have been helping train local soldiers to fight the Abu Sayyaf rebels in the area for several months.

Abu Sayyaf is best known for kidnapping for ransom, although the US has linked it to Osama Bin Laden's al-Qaeda network.

Jungle survival

The Americans were snatched from a luxury resort on 27 May last year, where they were celebrating their 18th wedding anniversary. Dozens of people were captured with them and in a series of raids that followed.

Some have been killed, including Californian Guillermo Sobero who was captured with the Burnhams, but most were released for ransom.

In a video taken in November, Martin said he and Gracia shared a hammock but "each night I am chained to a tree by my left wrist".

"Every time I hear a twig snap, I think it's a gunshot," Gracia said as her husband stood beside her.

Both looked in poor health and there were reports earlier this month that Martin Burnham was suffering from malaria.

Last month the kidnappers said they were no longer prepared to negotiate over the hostages and threatened to kill them if troops got too close.