The United States expressed outrage Thursday over the death of a leading Syrian doctor in what officials said appeared to be a targeted air strike.

State Department spokesman John Kirby said Doctor Hassan al-Araj had been a respected community leader and health professional in the northern town of Hama.

"The early indications are this doctor was deliberately targeted and killed, and all he was trying to do was save lives," he said.

Kirby did not say who was behind the "alleged air strike" but when asked who operates warplanes in the area said: "Well, the Russians do. The Russians do."

British-based monitor the Syrian Observatory for Human Rights said Araj died in an air strike near a hospital outside near Hama.

Reports identify Araj as a renowned cardiologist in his 40s who operated a hospital and field clinics in an area around Hama held by anti-regime rebels.

Kirby said "initial indications" showed the apparent strike hit the doctor "in a car, by himself, on a road in a remote area, nobody else around."

"He was a widely respected and beloved medical professional," the US spokesman said, praising the doctor for setting up clinics to treat war victims.

"Attacks against civilians, particularly medical professionals, are just abhorrent," he said.

"And we continue to call on everybody -- particularly the regime -- to respect the right of medical professionals to do their jobs and to save lives."

Syria's five-year civil war has dropped in intensity since a shaky ceasefire began in February and UN-mediated peace talks got under way in Geneva.

But the truce is violated daily and US officials say the most attacks are launched by Russian-backed regime forces loyal to Syrian President Bashar al-Assad.