A senior U.N. official earlier this month briefed the Security Council on reports that Syrian security forces had opened fire on defectors within their ranks and executed troops who refused orders to kill civilians, according to a copy of the notes used to give the confidential briefing.

The unverified reports cited by Oscar Ferandez-Taranco, the U.N. assistant secretary general for political affairs, appear to underscore the brutality of President Bashar al-Assad’s efforts to enforce loyalty within his ranks. The reports also raise questions about the veracity of the Syrian government’s claims, echoed by Brazil, India and Russia, that the violence in Syria is increasingly fought between two armed camps.

Syria’s U.N. ambassador, Bashar Jaafari, told reporters at U.N. headquarters last week that “gangs” and “terrorists” have killed more than 500 members of the Syrian security forces.

U.N. officials concede that it is impossible to obtain a clear picture of the events unfolding in Syria, where the government has made it difficult for journalists and other observers to report on the anti-government protest movement. But officials and experts say they believe the accounts of defectors, refugees and human rights groups that the government has mounted a bloody crackdown.

According to the Aug. 1 briefing notes, Taranco said that while “reports from a variety of sources assert that demonstrations have remained largely peaceful, there are also a few credible reports that military forces using excessive force against civilians have been confronted with armed opposition in some areas, in particular in Homs,” a city in western Syria.

“There are also indications that more than 300 security forces or army personnel have died, in circumstances that remain to be elucidated, but could include clashes with armed opposition as well as internal executions of defecting soldiers,” Taranco said. “A number of former defected soldiers or policemen interviewed by human rights organizations have stated that they received clear orders to use live ammunition against protesters and that those who did not obey were shot from behind by other security officers and Shabiha units,” a term for pro-government gunmen.

Taranco said there were similarities in attacks on civilian protesters. “The pattern of violent repression has remained the same, with military units, security forces and supporting militias using live ammunition, including from snipers, shelling from heavy machine guns and tanks as well as air power,” Taranco said. “Overall, human rights groups put the death toll at 1,500 killed.” The groups say that number has now surpassed 2,000.

Nadim Houry, a Beirut-based researcher for Human Rights Watch who has interviewed military defectors, said it is plausible that hundreds of members of the security forces have been killed during months of political unrest, including some who were killed by protesters who took up arms against the government.

“There have been a few examples where protesters did attack security forces,” he said. “But these have remained marginal to the larger narrative. The claim that the Syrian regime has been battling armed gangs and terrorists is frankly not true.”