* U.N. calls on Syria to halt use of force, mass arrests
* ICRC says force must be proportionate, seeks prison visits
(Adds interview with senior ICRC official)
By Stephanie Nebehay
GENEVA, May 13 (Reuters) - Up to 850 Syrians may have been killed in a two-month military crackdown and thousands of demonstrators have been arrested, the United Nations human rights office said on Friday.
"We again call on the government to exercise restraint, to cease use of force and mass arrests to silence opponents," Rupert Colville, spokesman of the U.N. High Commissioner for Human Rights, told a news briefing.
The toll of 700 to 850 dead, based on information provided by human rights activists, was "quite likely to be genuine", he said, adding it had lists of names of alleged victims.
Syria has blamed most of the violence on 'terrorist groups' backed by Islamists and foreign agitators.
Thousands of Syrians demonstrated after prayers on Friday, activists and witnesses said, keeping up a campaign of calls to end the autocratic rule of President Bashar al-Assad. [ID:nLDE74C179]
The International Committee of the Red Cross (ICRC) voiced concern at the Syrian government's use of force, saying it must be proportionate, and at difficulties for health workers to reach the wounded.
"We don't have a number for the dead and wounded, but various sources all point to a very high toll," Beatrice Megevand-Roggo, the ICRC's head of operations for the Middle East, told Reuters.
"There are rules for handling a demonstration and under international humanitarian law, the use of force must be proportionate," she said in Geneva.
The ICRC was seeking Syrian agreement to visit activists detained in the unrest. "We know there have been many, many arrests, certainly in the hundreds, probably thousands," Megevand-Roggo said.
SNIPERS, TANKS
The independent humanitarian organisation visits many detainees in the region, including in Jordan, Lebanon, Tunisia and Yemen, she noted.
The ICRC, believed to be the only foreign aid agency to have reached the southern town of Deraa, cradle of the uprising, has not been allowed back by authorities due to the security situation, Megevand-Roggo said.
During its May 5 mission, it delivered food, water, first aid kits and stretchers with the Syrian Red Crescent, which sent further supplies there this week, she said. [ID:nLDE7442DM]
"We are looking at what type of mobile medical structure we could put in place with the Syrian Red Crescent for situations where there is use of force and wounded," she said.
A U.N. human rights mission was preparing to go to Syria, as well as neighbouring countries, but had not yet received a reply from Damascus, Colville said.
"We have many reports of use of snipers, use of tanks in a number of towns. The government is reporting that soldiers and police have been killed, that is why we want to get in there and see for ourselves," he said.
The U.N. human rights office is also alarmed about the arrest and torture of dissidents in Bahrain, including the death of four detainees while in custody, Colville said.
"We continue to receive reports indicating that hundreds of individuals, including health professionals, politicians and human rights defenders arrested in connection with the protest movement are being denied their fundamental legal rights to due process," he said. (Additional reporting by Barbara Lewis; Editing by Michael Roddy and Janet Lawrence)
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