George Galloway, a maverick British lawmaker who had been acting as an intermediary for the family to bring Dr. Khan home, said Syrian authorities had informed him that Dr. Khan was found hanging by his pajama bottoms on Monday in his cell. Mr. Galloway, who had been planning to travel to Damascus, Syria’s capital, to escort the surgeon back to Britain, described the death as a tragedy and said, “I think we will have to wait for clarification on how exactly he died.”

The response was blunt from the British government, which supports the opposition seeking to topple Mr. Assad. Hugh Robertson, a minister in the Foreign Office, told the BBC that the surgeon’s death as described by his family would be extremely suspicious and “in effect” a murder. In an earlier statement, the Foreign Office said that if the surgeon had died in the custody of Syrian authorities, “responsibility for Dr. Khan’s death lies with them, and we will be pressing for answers about what happened.”

According to the BBC, the physician had been held at the headquarters of Syria’s national security agency, and his mother, Fatima Khan, had secured a promise of his release after having spent four months in Damascus.

When his mother saw him in prison on an earlier occasion, the family said, he weighed around 70 pounds, could hardly walk and said he had been tortured.

News of Dr. Khan’s death came as rights activists expressed new alarm over what they said were repeated Syrian military helicopter airstrikes in Aleppo, first reported on Sunday, in which crews dropped barrels filled with explosives and shrapnel onto rebel-held neighborhoods. Doctors Without Borders, the medical relief organization, added its concern on Tuesday, asserting in a statement that “despite inflicting widespread injuries and damage in civilian-populated areas, the indiscriminate and sustained attacks continued today.”