Syrian troops shelled the southern city of Daraa early today, killing at least 17 people, activists said, while in Damascus, residents spoke about a night of shooting and explosions in the worst violence Syria's capital has seen since the uprising against President Bashar Assad's regime began 15 months ago.

The nearly 12 hours of fighting in Damascus suggested a new boldness among armed rebels, who previously kept a low profile in the capital. It also showed a willingness by the regime to unleash in the capital the sort of elevated force against restive neighbourhoods it has used to crush opponents elsewhere.

For the first time in the uprising, witnesses said, regime tanks opened fire in the city's streets, with shells slamming into residential buildings.

The latest escalations in different parts of Syria are another blow to international envoy Kofi Annan's peace plan, which aims to end the country's bloodletting. Annan brokered a ceasefire that went into effect on April 12 but has since been violated nearly every day since and never properly took hold.

UN observers in the country ostensibly to monitor the ceasefire issued the first independent video images from the scene of a reported massacre last week in a remote farming village. Activists say up to 78 people, including women and children, were shot, hacked and burned to death in Mazraat al-Qubair on Wednesday.

The video, taken in the UN visit a day earlier, showed blood splashed on a wall pockmarked with bullet holes and soaking a nearby mattress. A shell punched through one wall of a house. Another home was burnt on the inside with dried blood was splashed on floors.

One man wearing a red-and-white checked scarf to cover his face, pointed at a 2008 calendar adorning a wall, bearing the photo of a lightly-bearded, handsome man. "This is the martyr," the resident, sobbing. He sat on the floor, amid strewn colourful blankets, heaving with tears. It was not immediately clear if he was a resident of the village or related to the man in the photograph.

"They killed children," said another unidentified resident. "My brother, his wife and their seven children, the oldest was in the sixth grade. They burnt down his house."

After the observers' visit, UN spokeswoman Sausan Ghosheh said the scene held evidence of a "horrific crime" and that the team could smell the stench of burned corpses and saw body parts strewn around the now deserted village, once home to about 160 people.

She said residents' accounts of the mass killing were "conflicting," and that the team was still cross checking the names of the missing and dead with those supplied by nearby villagers.

Opposition activists and Syrian government officials blamed each other for the killings. Activists accused pro-government militiamen known as "shabiha". A government statement on the state-run news agency SANA said "an armed terrorist group" killed nine women and children before Hama authorities were called and killed the attackers.

Thousands have been killed since the crisis began in March last year. The UN's latest estimate is 9,000 dead, but that is from April and it has been unable to update it. Syrian activists put the toll at more than 13,000.

The Damascus violence was a dramatic shift, since the capital has been relatively quiet compared with other Syrian cities throughout the uprising. Damascus and the northern city of Aleppo, the country's largest, are under the firm grip of Assad's security forces.

"Yesterday was a turning point in the conflict," said Maath al-Shami, an opposition activist in the capital. "There were clashes in Damascus that lasted hours. The battle is in Damascus now."

Blasts shook the neighbourhoods of Qaboun and Barzeh until about 1:30am today.

"We spent a night of fear," one resident said, speaking on condition of anonymity for fear of reprisals. The resident said the shooting and explosions in the capital "were the worst so far."

As tanks fired shells, troops clashed with rebels in the two neighbourhoods, al-Shami said via Skype. He said at least four people were killed.

The battles in the two neighbourhoods began during the day Friday, when troops opened fire on anti-Assad protest marches and rebels responded, witnesses said. In one brazen attack, the rebels struck a power plant in Qaboun with rocket-propelled grenades, setting fire to a generator and causing blackouts. The attack left buses charred and smashed a car. A video of the aftermath taken by UN observers said a soldier was killed in the RPG attack.

Also Friday, troops clashed with rebels from the Free Syrian Army in Damascus' Kfar Souseh district in fierce fighting sparked when the armed fighters attacked a military checkpoint in the area. The FSA, which groups defectors from the Syrian military with protesters who have taken up weapons, had made an unusually public appearance Thursday night in Kfar Souseh, overtly joining a large opposition rally. The bolder moves were a strong sign the ragtag group is pushing to take its fight to the regime's base of power.

To the south, regime forces heavily shelled a district of the city of Daraa until the early hours of today, smashing homes, according to activists. Daraa is the city where the uprising against Assad's regime first erupted in March 2011.

"People were taken by surprise while in their homes," Adel al-Omari, a local activist, said of the shelling, including mortar fire that hit the Mahata district.

The British-based Syrian Observatory for Human Rights said 17 people were killed in the shelling. The Local Co-ordination Committees said 19 civilians lost their lives, include a father and two children from one family and five members of another family.

The LCC and the Observatory also reported shelling and clashes in the central city of Homs, one of the main battlegrounds of the uprising. Both groups said troops stormed Homs' posh neighbourhood of Ghouta and the Observatory said security forces are conducting raids and searching for wanted people in the area.

In Turkey, Syria's main opposition council was scheduled later today to elect a new leader, nearly three weeks after its Paris-based president Burhan Ghalioun offered to step down over mounting criticism of his leadership. The executive committee of the Syrian National Council had asked Mr Ghalioun to pursue his duties until a new president is elected. His resignation last month came just days after he was re-elected for another three months in a controversial vote in Rome.

SNC spokeswoman Basma Kodmani said that the front-runner to replace Mr Ghalioun would likely be Abdulbaset Sieda, a member of Syria's minority Kurd community.