Ban Ki-moon, the United Nations secretary-general, has blamed the government of Bashar al-Assad, the Syrian president, for much of the "unacceptable levels of violence and abuses" occurring every day in the 14-month-long crisis in Syria.
In a report to the UN Security Council, issued on Friday, Ban cited the government's continued use of heavy weapons, reports of shelling and "a stepped-up security crackdown by the authorities that has led to massive violations of human rights by government forces and pro-government militias".
Ban, who is scheduled to brief the council on Wednesday, said there had only been "small progress" on implementing the six-point joint UN-Arab League plan brokered by international envoy Kofi Annan.
The UN chief called on the government to keep its pledge to immediately stop the violence, pull heavy weapons and troops out of populated areas, allow humanitarian workers to help civilians in need and end human rights abuses.
Ban also called on all elements of the opposition to stop the violence and respect human rights.
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The secretary-general said 271 of 300 unarmed UN military observers authorised by the council to help end the conflict were on the ground.
Their deployment in key cities appeared to have had a "calming effect", he said.
Nonetheless, "the overall level of violence in the country remains quite high"' with daily incidents causing a large number of deaths and injuries, though at a lower scale than immediately before April 12 when a ceasefire was supposed to take effect.
"While the international effort is making some impact on the ground, unacceptable levels of violence and abuses are continuing in violation of ... the six-point plan,"' Ban said.
He said the situation posed "serious challenges"' for the UN mission and the observers, who had been threatened and targeted, with their vehicles damaged and their movements restricted by crowds.
"This is a source of grave concern, and underscores the need to carefully consider the United Nations presence and next steps, taking into account the volatile and evolving security environment," Ban said.
Violence continued in the country on Friday with at least 50 people, including 13 children, killed after government loyalists tried to break into the town of Houla in Homs province, activists said.
An amateur video posted online by activists showed more than a dozen bodies lined up inside a room. They included about 10 children who were covered with sheets that only showed their bloodied faces.
The London-based Syrian Observatory for Human Rights said in one incident in Houla, a family of six was killed when their home received a direct hit.
Elsewhere in the country, Syrian forces fired tear gas and live ammunition to disperse thousands of protesters in Aleppo calling for an end to Assad's rule, killing several people, activists said.
Mohammad Saeed, an Aleppo-based activist, said more than 10,000 people were protesting in the city.
"The regime is desperately trying to put down the protests in Aleppo but all this violence will backfire," he said.
He added that security forces had shot dead five people, including a 12-year-old boy, identified as Amir Barakat.