The spread of Syria's civil war has made it increasingly difficult for civilians to escape the conflict, and many are afraid to seek medical care, the head of the International Committee of the Red Cross said on Thursday.

"Through the spreading of the fighting people lose ... escape routes out of the fights," Peter Maurer told reporters in Stockholm after a meeting with Sweden's Development Aid Minister Gunilla Carlsson.

"In summer, when fighting was going on in Aleppo and Homs, you could still move to Idlib or to some other places. Those places are increasingly rare because fighting is covering more parts of Syria."

The latest toll brought the number of people killed in 20 months of violence in the country to more than 40,000, according to the Syrian Observatory for Human Rights.

Maurer also warned that "a much deeper humanitarian crisis" would unfold if attacks on medical workers and ambulances continued.

"The general population of Syria is afraid to see medical doctors and go to hospital because hospitals have become" military targets, he said.

Although the aid agency has managed to double the amount of aid brought into the country over the past three months, it was sometimes difficult to negotiate access for "neutral, impartial" deliveries in the highly polarised country, he added.

The Syrian uprising began as peaceful reform protests last year, inspired by the Arab Spring. It has since been transformed into an armed insurgency after the government began crushing demonstrations.

Most rebels, like the population, are Sunni Muslims in a country dominated by a minority regime of Alawites, an offshoot of Shiite Islam.