British Prime Minister David Cameron paid tribute to the kidnapped British Red Cross worker found dead in Pakistan, calling his murder a "shocking and merciless act".
Cameron said he "was deeply saddened" to hear about the death of Khalil Dale, and accused the attackers of having "no respect for human life and the rule of law."
"This was a shocking and merciless act," added the leader.
"Khalil Dale has dedicated many years of his life to helping some of the most vulnerable people in the world and my thoughts today are with his friends and family," he said in a statement.
London had tried tirelessly to secure the release of the 60-year-old since his kidnapping in Baluchistan province in January, British Foreign Secretary William Hague said in an earlier statement.
"This was a senseless and cruel act, targeting someone whose role was to help the people of Pakistan, and causing immeasurable pain to those who knew Mr Dale," said Hague.
The Red Cross announced earlier Sunday that Dale, a health programme manager, had been found dead in Quetta, Pakistan, four months after he was abducted by armed men.
The mutilated body of Khalil Dale, 60, was dumped in a bag in an orchard on the outskirts of Quetta, the capital of the insurgency-hit southwestern province of Baluchistan.
A note claiming to be from militant group Tehreek-e-Taliban Pakistan was found with the body, senior local police official Tariq Manzoor told AFP.
The group said in the note that "our demands were not met (and) we have stuffed his (Dale's) body in a bag after slaughtering him. We will soon release a video of his beheading," according to Manzoor.
Dale, a British Muslim who had been managing a health programme in Quetta for almost a year, was abducted in the city on January 5 by eight masked gunmen, who forced him from his car at gunpoint as he returned home from work.
A source close to the case said the captors had demanded a ransom of $30 million.
The International Committee of the Red Cross "condemns in the strongest possible terms this barbaric act", said its Director-General Yves Daccord.
"All of us at the ICRC and at the British Red Cross share the grief and outrage of Khalil's family and friends."
"We are devastated," Daccord said, adding that the aid worker -- who had worked in Somalia, Afghanistan and Iraq -- was a "trusted and very experienced Red Cross staff member".
Pakistan's government also condemned "the barbaric act" and vowed "to bring perpetrators of this heinous crime to justice".
Riffat Hussain, a police surgeon at Bolan Medical College in Quetta, where the aid worker's body was taken, told AFP that Dale had been decapitated, but doctors had stitched his head back on to his body.
He is believed to have been killed some 12-14 hours before his body was discovered, the surgeon said.
"There were no signs of any torture on his body but different body joints were dislocated" when he was stuffed into the bag, the surgeon added.
Police official Manzoor told AFP: "His body was found in a bag in an apple orchard on the outskirts of Quetta on Sunday morning."
He said police had difficulty identifying Dale as he had grown a beard, but staff from the ICRC had confirmed the remains were his.
Baluchistan is plagued by a separatist insurgency, sectarian violence and Taliban militants.
Local rebels rose up in 2004 demanding political autonomy and a greater share of profits from the region's natural oil, gas and mineral resources.
Kidnappings are rife in parts of the province, where criminals looking for ransoms snatch foreigners and locals, sometimes passing their hostages on to Taliban and Al-Qaeda-linked groups.
In February 2009, John Solecki, the local head of the UN refugee agency (UNHCR), was snatched at gunpoint in Quetta, with his driver killed during the abduction. Solecki was released after nearly nine weeks.
In Baluchistan, the ICRC mainly focuses on health programmes and supports several medical centres, including a hospital.
The ICRC had announced a reduction of its activities in Pakistan just days before Dale's abduction with the closure of three of its centres in the restive northwest.
But after the kidnapping the organisation vowed to continue its work in the troubled country.© ANP/AFP