BEIRUT (Reuters) -- A Red Crescent volunteer was shot dead on first aid duty in Syria, his organization said on Saturday, the fourth local aid worker to be killed as unrest grows increasingly bloody.

Bashar al-Youssef, 23, was shot in the head on Friday while wearing a uniform clearly marked with the Syrian Arab Red Crescent emblem, the SARC said in a statement.

"We are shocked by Bashar's death, it is completely unacceptable," said Abdul Rahman al-Attar, the president of the Syrian Arab Red Crescent.

Youssef had been working in Deir Ezzor, a flashpoint eastern province where civilians have been affected by heavy shelling and fierce fighting between security forces and rebels who have joined the uprising against President Bashar Assad's rule.

The identity of Youssef's killer was not clear.

Red Crescent workers have often been caught in the line of fire, and are sometimes viewed with distrust by both Assad loyalists and the opposition, who are suspicious of the group's neutrality in a conflict increasingly described as civil war. SARC workers say they have been targeted by both sides.

The International Committee of the Red Cross stressed the importance that Syrian aid workers not be harmed in a joint statement with the SARC.

"This comes at a time when the ICRC and the Syrian Arab Red Crescent are virtually the only organizations able to work in areas affected by the violence in Syria," said Alexandre Equey, the deputy head of the ICRC delegation in Syria.

Syrian Red Crescent workers mourning Youssef started a Facebook awareness campaign, using a picture of a smiling young man in his red aid uniform next to pictures of the three volunteers killed before him. A fifth picture of a uniformed worker was shown with a blank face and a question mark, labeled "the next martyr".