UN special adviser on the prevention of genocide, Francis Deng, and special adviser on the “responsibility to protect,” Edward Luck, in a joint statement said they were “gravely concerned at the increasing loss of life in Syria (due to) ... continued violent suppression of anti-government protests.”

“We are particularly alarmed at the apparently systematic and deliberate attacks by police, military, and other security forces against unarmed civilians taking part in the last two months of protests,” Deng and Luck said.

Human rights activists said Syrian forces killed at least 13 civilians in the central town of Rastan on Thursday, in the latest attempt to quell a revolt against the 11-year rule of President Bashar Al-Assad.

Syria has barred most international media, making it difficult to verify accounts of the violence.

The UN Security Council is considering a draft resolution condemning Syria which was circulated to its 15 members by Britain, France, Germany and Portugal last week. Veto powers Russia and China have made clear they dislike the idea of council involvement in what they see as a domestic issue.

Diplomats said Security Council envoys were meeting on Thursday to see if there was any way to amend the resolution so that Russia and China would not veto it.

Deng and Luck said that Syrian forces appear to have been targeting residential neighborhoods in their operations.

“The deployment of armed forces and the use of live fire, tanks and artillery in response to peaceful protests, and the targeting of residential areas where protests have taken place, are unacceptable under any circumstances,” Deng and Luck said.

Syria blames the unrest on armed groups backed by Islamists and foreign powers. Assad has sent security forces and tanks to several protest flashpoints, including Daraa, Banias and Tal Kelakh, a border town near Lebanon, and now Rastan.

Deng and Luck said the Syrian authorities may be guilty of a number of rights abuses.

“In addition to the deaths of protesters, there have also been reports of mass arrests, arbitrary detentions, disappearances and other serious human rights violations in the towns where protests have taken place,” they said.

Rights groups estimate that over 10,000 people have been rounded up and detained since protests erupted in mid-March.

Deng and Luck called for “an independent, thorough, and objective investigation into all alleged violations of international human rights law.”