DAMASCUS (AFP) - Syria said on Wednesday that a British doctor alleged to have been effectively murdered in custody had hanged himself after being arrested for "unauthorised activities."

Meanwhile, regime warplanes pounded Aleppo for a fourth straight day in raids that have killed at least 189 people and overwhelmed the northern city's hospitals, Doctors Without Borders (MSF) said, citing local medics.

The death of Dr Abbas Khan just days before he was to be handed over to a British lawmaker sparked a diplomatic row with London, which accused President Bashar al-Assad's regime of effectively murdering the 32-year-old orthopaedic surgeon.

But state news agency Sana said Dr Khan had entered Syria illegally and undertaken "unauthorised activities." It also cited a medical report as saying his death by hanging was "committed by the person himself with the aim of taking his own life" and that tests had shown "no trace of violence or resistance." Syrian Deputy Foreign Minister Faisal Meqdad had said on Tuesday that Dr Khan hanged himself with his pyjamas.

Dr Khan, a volunteer with London-based charity Human Aid UK, had travelled to Aleppo last year to help civilians but was captured by the regime.

His family announced on Tuesday that he had died in detention and said they were "shocked and devastated," as Syrian authorities had promised to release him this week.

Dr Khan's brother Shahnawaz said the government's offer earlier on Wednesday to arrange an independent autopsy was "frankly insulting." Mrs Fatima Khan, the doctor's mother, pleaded with Syria to return his remains, saying in an interview with ITV News: "If you can't give me him alive, at least give me his dead body." Syria's regime has been accused of torturing and killing an untold number of prisoners during the nearly three-year uprising, in which an estimated 126,000 people have been killed and millions displaced.

The Syrian Observatory for Human Rights, a monitoring group which relies on activists, medics and other witnesses, said it had documented "hundreds" of cases in which the regime said prisoners committed suicide when in fact they were tortured to death.

Jihadist groups fighting alongside the rebels have also been accused of torturing and executing captives, and on Wednesday fighters with the Islamic State of Iraq and the Levant (ISIL) - an Al-Qaeda affiliate - beheaded three men they said were Alawites, members of Assad's religious sect, an offshoot of Shiite Islam.

The Observatory said the men were from the ethnically and religiously diverse town of Adra near Damascus, the scene of fierce fighting in recent days. Jihadist websites posted gory footage of the heads placed next to the bodies.

Death toll rises in bombardment of Aleppo

In Aleppo, Syria's second city and onetime commercial hub, a regime bombing campaign continued for a fourth day, with helicopters and other aircraft dropping crude barrel bombs filled with TNT onto rebel-held enclaves, the Observatory said.

Doctors Without Borders (MSF) said the air strikes had killed 189 people and wounded another 879, including 244 children. The Observatory on Tuesday gave a toll of 135 killed.

One Syrian security source denied that barrel bombs have been used against what the regime calls "terrorists," but another said the military prefers such weapons over missiles because they are cheaper.

The opposition National Coalition has said the "systematic raids on Aleppo demonstrate the regime's rejection of a political solution." A UN-backed peace conference dubbed Geneva 2 is due to be held in the Swiss city of Montreux on January 22.

A senior opposition official insisted Wednesday that Western nations still insisted Assad play no role in any political transition, after press reports suggested they may want Assad or his inner circle to remain in government, fearing the rising power of jihadist rebels.

"The 11 countries (including the United States, France and Britain), and behind them the 100 members of the 'Friends of Syria,' agree that there should be no role for Assad," Monzer Aqbiq told AFP, referring to discussions last week in London.

In another development, the Organisation for the Prohibition of Chemical Weapons said Russian armoured trucks tracked by US satellite equipment and Chinese cameras would help ship Syria's weapons out of the country.

The details were spelled out in a blueprint released Wednesday by the OPCW, charged with implementing a UN-backed plan to eradicate Syria's chemical arsenal by mid-2014.