Syrian authorities moved hundreds of detainees to military sites off-limits to Arab League monitors, Human Rights Watch said on Wednesday, as observers began a mission to assess whether Syria has ended its violent crackdown on anti-government protests.

A report by the U.S.-based group urged the observers, who began their mission in the flashpoint city of Homs, to insist on full access to all sites used for detention.

The report issued late on Tuesday is based on interviews with a Syrian security official and witnesses, including a detainee and residents in Homs who said they saw heavily guarded buses leave several prisons, leading them to suspect that they were moving detainees.

The security official told HRW that he received orders to assist with an "irregular detainee transfer" after Damascus agreed to admit the monitors under a deal which calls for an end to violence, the withdrawal of troops from the streets, the release of prisoners and dialogue with the opposition.

The report also said that police identification cards had been issued to many soldiers to give the appearance that police and not the military were patrolling the streets.

A Syrian security official told HRW he estimated that on Dec. 21 and 22 approximately 400 to 600 detainees were moved out of his detention facility to other places of detention.

"The transfers happened in installments. Some detainees were moved in civilian jeeps and some in cargo trucks," the official said.

He said that officials who accompanied the detainees out of the facility told him they were being taken to a military missile factory in Zaidal, just outside Homs.

The report comes as Syrian authorities released 755 prisoners who were detained during ten months of anti-government protests, the state-run news agency SANA said on Wednesday.

The agency said prisoners with "blood on their hands" were not among those released. The move comes a day after Arab League observers started to tour the country.

Syrian security forces have released at least 1,000 people but activists say tens of thousands remain in detentions around the country.

It is difficult to verify events as most foreign journalists are banned from the country.

"My role was inside the prison, gathering the detainees and putting them in the cars. My orders from the prison director were to move the important detainees out," the security source told HRW. 

Meanwhile, Russia, one of Syria's few remaining allies, urged Damascus on Wednesday to allow members of an Arab League mission to move around the country freely as they investigate whether Syria is implementing a peace plan to end months of violence.

"The mission should be able to visit any part of the country, any towns or villages, and come up with its own independent, objective opinion about what is happening and where," Foreign Minister Sergei Lavrov said at a news conference with his Egyptian counterpart.

Russia has submitted a revised United Nations draft resolution condemning the bloodshed but stopped short of blaming Syrian President Bashar Assad. Moscow says it supports the observer mission.

"We have consistently worked with the Syrian leadership, urging full cooperation with the observers on the provision of comfortable and free conditions for work," said Lavrov.

The group will assess whether Assad is keeping a promise to withdraw troops from cities and halt violence that threatens to spiral into civil war.

The 21-member group of Arab states has threatened sanctions if the crackdown continues -- a punishment Moscow rejects warning that any arms embargo would cut off Damascus from weapons, while protesters attain weapons, helping to escalate the conflict.

Syria accounted for seven percent of Russia's total of e10 billion in arms deliveries abroad in 2010, according to the Russian defense think tank CAST. Russia also maintains a naval maintenance base in Tartus.

After Western-backed military intervention helped topple Libyan leader Muammar Gaddafi this year, Moscow has stridently opposed what it calls "external interference" in North Africa and the Middle East.

Meanwhile, Germany has asked the Syrian ambassador to Berlin to say if he knows anything about an attack on an opposition activist in the German capital, a government spokesman said Wednesday.

The ambassador had been asked to the Foreign Ministry for a conversation, but was not being "summoned," the Berlin spokesman said.

Police are investigating the early Monday attack on Ferhad Ahma, who is an activist with both the German Green Party and the opposition Syrian National Council. Ahma said two men entered his home in the night and attacked him.

Green Party lawmaker Volker Beck, who has charged that the attackers were agents of the regime of President Bashar al-Assad, demanded earlier that the ambassador be summoned and handed an official protest.

A Foreign Ministry spokesman said the diplomat would be asked if he knew anything about the matter.

He said any form of intimidation was not acceptable to Germany, adding that there would be a rapid response if it became evident that individuals with diplomatic accreditation were involved in violence, such as declaring them persona non grata, forcing their departure.

Ahma also called for the Foreign Ministry to condemn the incident.

"That would serve to protect other opposition activists here in Germany," Ahma told dpa in an interview.

"It has to be clear that such acts cannot be tolerated in this country," he said.