BEIRUT (AP) — More than 600 medical workers have been killed in Syria's civil war in deliberate and indiscriminate attacks, most of them by government forces, an international rights group announced on Wednesday.

Physicians for Human Rights said it has documented 233 attacks on 183 medical facilities across Syria since the country's conflict began in March 2011. In a report, it said that President Bashar Assad's government is responsible for 88 percent of the recorded attacks on hospitals and 97 percent of the killings of medical workers.

It documented 139 deaths directly attributable to torture and execution.

The group's director of investigations Erin Gallagher said, "every doctor killed or hospital destroyed leaves hundreds or even thousands of Syrians with nowhere to turn for health care."

The health system in Syria has been devastated by the country's civil war, now entering its fifth year.

Separately, Medecins Sans Frontieres (MSF) said Wednesday that desperately-needed aid is failing to reach millions of people who are trapped by the conflict, and a massive increase in medical assistance is required.

From an estimated 2,500 doctors who worked in Aleppo, Syria's second biggest city, before the conflict, less than a hundred remain in the hospitals still operating in the city, MSF said. The rest have fled, become internally displaced, or have been kidnapped or killed.

The gradual deterioration of the security situation and the abduction of five staff members by the Islamic State group in January 2014 forced MSF to reduce its own activities in Syria.

"Not only did this serious incident precipitate the closure of health facilities in IS-controlled areas, it also meant that most of MSF's international medical staff could no longer work in Syria, as we could no longer trust that our teams would not be harmed," said Joanne Liu, MSF's International President.

She called for a large-scale international humanitarian effort in Syria.

"We can and should do better for the people of Syria," Liu said.