GENEVA: The International Committee of the Red Cross (ICRC) is evacuating some expatriate aid workers from Syria due to the worsening security situation, moving them temporarily to Beirut, a spokesman said Friday.

Such withdrawals are rarely announced by the independent agency, whose staff have crossed front lines throughout the 16-month-old conflict. The ICRC's move follows a partial evacuation by U.N. aid workers earlier this week.

"An unstable and deteriorating situation in several parts of the country has led the ICRC to temporarily relocate some of its staff outside of Syria," ICRC spokesman Hicham Hassan said in Geneva.

Some staff would be moved over the next few days but continue to support the ICRC delegation of 50 expatriates and nationals remaining in Damascus, he told Reuters.

"They will be going back to Damascus as soon as the situation allows. A team of expatriates and nationals will continue working in Damascus."

The ICRC had been building up its aid operation and along with the Syrian Arab Red Crescent delivers the bulk of vital food and medical supplies reaching civilians in flashpoints including Homs, Hama, Idlib, Deraa and rural areas around Damascus.

The United Nations, which mainly delivers aid through the Syrian Arab Red Crescent, quietly halved its humanitarian operation to 30 staff in the week since a July 18 bomb in Damascus killed four senior aides of President Bashar al-Assad.

The United States said on Thursday it feared a massacre would occur in Aleppo after Assad's forces launched ground and air bombardments in preparation for a major onslaught against rebels in Syria's commercial capital.

U.N. human rights chief Navi Pillay appealed to both Syrian government forces and rebels on Friday to spare civilians in Aleppo, voicing deep concern at the "likelihood of an imminent major confrontation" in the city.

The Syrian Arab Red Crescent has suspended some of its aid operations in Aleppo, mainly the movement of ambulances and first aid work by volunteers, Hassan said.

That decision, taken on Wednesday, reflected a tense security situation and lack of respect for Red Crescent staff, structures and ambulances, he said.

The Red Crescent's other 17 branches scattered throughout the battered country continue to operate, he said. Five of the organisation's aid workers have been killed in the conflict.