(CNN) -- Syrian security forces have committed "gross violations of human rights" since anti-government protests began in March, a United Nations report released Monday said.

The U.N. independent international commission on Syria's assessment was based on interviews with 223 victims and witnesses, but observers were not allowed access to the country, the report said.

According to the report, evidence "documents patterns of summary execution, arbitrary arrest, enforced disappearance, torture, including sexual violence, as well as violations of children's rights."

Military deserters told the commission that they had been ordered to shoot at unarmed protesters without warning, the report said.

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"The commission reiterates its call for immediate and unhindered access to the Syrian Arab Republic," the report said.

Earlier this month, the United Nations High Commission for Human Rights estimated that government forces had killed at least 3,500 civilians.

Syria's government has consistently blamed armed gangs for the violence and said security forces are protecting the people.

"The Arabic Syrian military, which we are all proud of, has given martyrs in order to protect the life of civilians," Syrian Foreign Minister Walid Moallem said Monday.