The body of a British doctor who died in Syrian custody was handed over to his family and British officials in Lebanon Saturday as relatives said the regime killed him.
Human Rights Watch also piled the pressure on Syrian authorities, accusing the government of "wreaking disaster" in deadly air raids on second city Aleppo.
On the diplomatic front, peace envoy Lakhdar Brahimi held talks with the foreign minister of Syria's ally Iran, after negotiators failed to agree on a role for Tehran in a peace conference.
The International Committee of the Red Cross (ICRC) said Doctor Abbas Khan's body was brought out of Syria and given to his family and British officials in Beirut.
"The British embassy in Lebanon is expected to rapidly fly the body to London," a statement said.
Khan's mother, Fatima, was at a Beirut hospital morgue to receive his remains and accused Syrian authorities of killing him.
Britain also holds Damascus responsible for the death of the 32-year-old orthopaedic surgeon.
Khan, a volunteer with Human Aid UK, had travelled to Aleppo last year to help civilians when the regime arrested him.
Syria said this week that Khan had been found "hanging" in his cell, where he was being held for "unauthorised activities," and that he had committed suicide.
But his mother told the BBC that Syrian security authorities had killed him.
"How come they can't differentiate between a humanitarian aid worker and a terrorist?" she asked.
"He was so good to humanity. How could they kill a humanitarian aid worker? He did not treat any terrorists, he treated only women and children, he told me."
Khan, who also visited Syria to try to secure his release, said her son had told her: "Look I am a doctor mummy. My profession is to give life," not to take it.
While he was in detention, Syrian television had described Khan as a "terrorist," the term it uses to brand its opponents.
'Wreaking disaster in Aleppo'
Syria's 33-month war has killed an estimated 126,000 people, and Human Rights Watch says air force bombing raids on Aleppo have left hundreds more dead in the past month.
"The Syrian air force is either criminally incompetent, doesn't care whether it kills scores of civilians -- or deliberately targets civilian areas," said HRW's Ole Solvang.
The statement comes six days after the launch of a massive aerial campaign against rebel-held areas of Aleppo, during which activists said TNT-packed barrels were used.
HRW cited the Syrian Network for Human Rights as saying 232 civilians were killed in and around the city on December 15-18.
The Syrian Observatory for Human Rights said strikes on Saturday killed at least nine civilians.
In Daraa in the south, where opponents say they have scored recent advances, the air force dropped TNT-packed barrels on Inkhil, said the Observatory.
Warplanes had also bombarded a Palestinian refugee camp in Daraa city, killing nine people, among them two women and an army defector.
Abu Anas, an activist in the province, told AFP via the Internet that "the air force has for three days been dropping barrel bombs on Inkhil, Daraa al-Balad (in Daraa city) and Jassem. It's targeting civilian areas."
The violence comes despite preparatory talks for a long-awaited Syria peace conference slated to take place in Switzerland on January 22.
The conference organised with backing from Russia and the United States is due to bring together opposition and regime representatives.
Brahimi unveiled Friday a list of countries and organisations that are to attend the conference. Iran was not among them, but Saudi Arabia, which backs the rebels, was invited.
On Saturday the envoy discussed preparations for the peace talks with Iranian Foreign Minister Mohammad Javad Zarif.
Zarif "insisted on a political solution" to help end the conflict, his ministry said.
Brahimi said Friday that Iran was not yet "off the list" of participants.
Tehran is accused of providing Syria financial and military aid and is a key backer of Lebanon's Hezbollah Shiite movement, whose militants have been fighting alongside loyalist forces.
The opposition National Coalition repeated Saturday its rejection of a role for Iran in the talks.
"We consider Iran to be one of the main responsible forces for the criminal gang that is in Syria continuing to kill the Syrian people," said spokesman Khaled Saleh.