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 government forces 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 next month.
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 present at a Beirut hospital morgue to receive her sons remains and lashed out at Syrian authorities, saying they had killed him.
Britain also holds Damascus responsible for the death of the 32-year-old orthopaedic surgeon.
Khan, a volunteer with London-based charity Human Aid UK, had travelled to Aleppo last year to help civilians when the regime arrested him.
Syrian authorities said Wednesday that Khan was had been "hanging" in his cell, where he was being held for "unauthorised activities," and that he had committed suicide.
But his mother said Syrian security authorities killed him.
"How come they can't differentiate between a humanitarian aid worker and a terrorist," she told the BBC.
"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," said Fatima Khan.
Khan, who also visited Syria to try and secure his release from jail, said her son had told her once: "Look I am a doctor mummy. My profession is to give life," not to take it.
While he was still in detention, Khan was described on Syrian television as a "terrorist," the term used by Damascus to brand all its opponents.
Fatima said arrangements had been made to take the body to a coroner in London, and the Metropolitan Police have said they would assist the coroner "when appropriate".
'Wreaking disaster in Aleppo'
Syria's 33-month war has killed an estimated 126,000 people and Human Rights Watch says the air force has killed hundreds more men, women and children in bombing raids on Aleppo 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 opposition-held areas of the northern city, 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 Aleppo on December 15-18.
The Syrian Observatory for Human Rights said new strikes Saturday killed at least nine civilians.
The violence comes despite preparatory talks ahead of 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 who are to attend the conference. Damascus ally Iran is not among them, but Saudi Arabia which backs the rebels has been 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 negotiators had failed to reach agreement on whether Iran should be invited to the talks, but that it 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 the rebels alongside loyalist forces.
Analysts have said Hezbollah and Iran's involvement in Syria have helped regime troops advance towards Damascus and Aleppo.
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.