United Nations observers arrived in the Syrian town of al-Haffeh Thursday after government forces overran the opposition enclave, while Syria's envoy to Moscow denied that Russia is supplying Syria with attack helicopters.

The U.N. monitors had been trying to enter al-Haffeh after several days of intense clashes. They found the Sunni Muslim town nearly deserted, with state buildings burnt, shops abandoned and a corpse lying in the street.

Syrian authorities on Wednesday said they had “cleansed” the area of armed terrorist groups. On Tuesday, a mob outside the town had attacked the U.N. observers' vehicles with rocks and metal rods, forcing them to turn back.

Anti-government rebels pulled out of the town this week, and joined the United States in warning that some of the people remaining in al-Haffeh could be subject to reprisal killings.

Violence continued elsewhere in Syria on Thursday. Reuters news agency reported Syrian forces fired heavy artillery on the eastern city of Deir el-Zour, killing at least 11 people and wounding about 200. Opposition sources said Free Syrian Army fighters have killed dozens of troops and destroyed several tanks and armored personnel carriers there in the past week.

A suicide bomber detonated an explosives-packed van in a Damascus suburb, wounding 14 people and damaging one of Shi'ite Islam's holiest shrines. Government troops continued to pound rebel-held areas in the central city of Homs and other areas.

Meanwhile, the Syrian ambassador to Russia denied U.S. Secretary of State Hillary Clinton's remark that Moscow is shipping attack helicopters to his country. Riyad Haddad said the arms deliveries are defensive weapons. He blamed Western countries for any failures of international envoy Kofi Annan's peace plan.

Rafif Jouejati, a spokeswoman for the Local Coordination Committees, an activist group with members throughout Syria, told VOA that helicopter strikes and other aerial attacks by government forces have been occurring for months across Syria.

Chinese Foreign Ministry spokesman Liu Weimin said Thursday his country does not approve of using sanctions to address the crisis in Syria.

“Under the current circumstances, all the parties should continue to vigorously support U.N. envoy Kofi Annan's mediation efforts. We urge relevant parties in Syria to effectively implement relevant U.N. Security Council resolutions and Annan's six-point proposals, actively cooperate with the U.N. monitors, end any form of violence, protect civilians, and ease the current tense situation as soon as possible.''

Amnesty International says it has new evidence of widespread and systematic rights violations by government forces seeking to punish those supporting the opposition.

The group says its workers witnessed Syrian security forces firing on peaceful demonstrators late last month in Aleppo, and that families described soldiers dragging away family members and killing them.

Donatella Rovera, Amnesty International's senior crisis adviser, said the U.N. Security Council has failed, and called for “concrete action” to hold those responsible accountable.

“We are now facing a situation which has deteriorated so much precisely because of the failure of the Security Council to act earlier on when the situation was — when it was more possible to avoid the kind of large scale killings and massacres that we're seeing today.''

The rights group said it has received reports of more than 10,000 people being killed since the crisis in Syria began in February 2011, and that the number could be much higher.