A suicide bomber blew up a vehicle near an important Shiite shrine in Syria's capital Thursday, wounding 14 people, state media and witnesses said, as 35 people were reported killed across the country.

Official news agency SANA said the vehicle exploded in a garage 50 metres (yards) from Sayyida Zeinab shrine. There was "substantial damage in the area of the blast" and "the terrorist who carried out the operation was killed," it said.

The Syrian Observatory for Human Rights, citing anti-regime activists, said the bomb went off near security offices, damaging the apparent target as well as the shrine, as seen in a video posted on the Internet.

A witness said a van drove at speed into the parking lot at 6 am (0300 GMT) and exploded among parked vehicles, including pilgrim buses. The vehicles and a police station were damaged, an AFP photographer said.

The windows of the mausoleum were shattered and its air vents ripped out by the blast, which left a three-metre (10-foot) crater. Tiles on the minarets were damaged.

International peace envoy Kofi Annan has warned that Syria's nearly 16 months of deadly unrest could become all-out sectarian war.

Most of Syria's 22-million population are Sunni Muslims, while its minorities include Alawites, an offshoot Shiite community to which President Bashar al-Assad belongs.

Hundreds of thousands of pilgrims, mainly from Syria's ally Iran, travel each year to the shrine of Sayyida Zeinab, a granddaughter of the Muslim Prophet Mohammed, in an area of south Damascus that is home to many Iraqi refugees.

Separately on Thursday, a car bomb in the northwestern city of Idlib targeted a military checkpoint, the Observatory said, adding that an unknown number of soldiers were killed or wounded.

At least 35 people were killed in violence across the country, it added.

UN observers, meanwhile, visited Al-Haffe town in the Mediterranean province of Latakia, a day after Syrian authorities said the area had been "cleansed" of rebel fighters, a UN spokeswoman in Damascus said.

On Wednesday, rebels withdrew from the besieged town and nearby villages that had been under intense regime shelling for eight days, the Observatory said.

The UN Supervisory Mission in Syria (UNSMIS) said observers in Al-Haffe reported finding it all but deserted with a strong stench of dead bodies and most state buildings gutted.

"Most government institutions, including the post office, were set on fire from inside," a statement said. "Archives were burnt, stores were looted and set on fire, residential homes appeared rummaged and the doors were open."

It added that "a strong stench of dead bodies was in the air and there appeared to be pockets in the town were fighting is still ongoing."

It said the number of casualties was still unclear.

State television said the observers had "inspected the vandalism and destruction wrought by the terrorists."

The United Nations and opposition activists had expressed fears of a massacre if pro-government forces entered the town, just 16 kilometres (10 miles) from Assad's mainly Alawite hometown of Qardaha.

As the death toll soars, Amnesty International accused Syria of committing crimes against humanity to punish communities supporting rebels.

The London-based group called for an international response after claiming it had fresh evidence that victims, including children, had been dragged from their homes and shot dead by soldiers, who in some cases then set the bodies on fire.

"This disturbing new evidence of an organised pattern of grave abuses highlights the pressing need for decisive international action," said Amnesty's Donatella Rovera on the release of the 70-page report entitled Deadly Reprisals.

The group interviewed people in 23 towns and villages and concluded that government forces and militias were guilty of "grave human rights violations and serious violations of international humanitarian law amounting to crimes against humanity and war crimes."

The allegations came as the Observatory reported that more than 14,400 people have been killed in Syria in the revolt against Assad's regime, including 2,302 in the past month alone.

Opposition sources said anti-Assad groups are to meet in Istanbul on Friday and Saturday in a bid to settle their differences and close ranks.

Senior members of the main opposition Syrian National Council, the Kurdish National Council and smaller groups such as one led by tribal chief Nawaf al-Bashir are to take part, they said.

"It is kind of a last call to join us," an SNC source said, on condition of anonymity.

On the diplomatic front, British Foreign Minister William Hague urged Russia and Iran to use their "full influence" over ally Syria to achieve a peaceful end to the bloody uprising.

Hague met his Russian and Iranian counterparts in Kabul on the sidelines of a conference on Afghanistan.

China, meanwhile, said it disapproved of "one-sided" sanctions and pressure on Syria after France raised the prospect of a new raft of punitive measures against Assad's regime.

In other developments, UNSMIS said observers monitored the release of 239 detainees in Damascus, and another 13 in Idlib and Deir Ezzor.

It said the government had told it a total of 500 are being freed nationwide.