Heavy clashes between rebels and regular troops have erupted in Damascus in the "most intense" fighting in the
capital since the start of the anti-regime revolt in Syria 16 months ago, a monitoring group has said.
"The regular army fired mortar rounds into several suburbs" where fighters of the Free Syrian Army are entrenched, said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights on Sunday.
"They have never been this intense," Abdel Rahman told the AFP news agency.
He said the fighting was heaviest in the Tadamon, Kfar Sousa, Nahr Aisha and Sidi Qadad neighbourhoods.
"The security forces are attempting to take control of these neighbourhoods, but so far they have not succeeded," he added.
The Local Co-ordination Committees, which organises anti-regime protests in Syria, said plumes of black smoke were Sunday night billowing out of Tadamon and that loud explosions were heard in Nahr Aisha.
The Observatory earlier said violence across Syria on Sunday killed at least 55 people, including a girl who died along with three other people when the army rained shells on the town of Rastan, a rebel stronghold in the central province of Homs.
Another group of UN observers headed to Douma on Sunday in order to hold a humanitarian visit to hospitals and schools and assess the effect of the 16-month crisis in the country.
Activists said that at least nine people were killed earlier on Sunday, including two civilians when troops shelled the flashpoint city of Homs as they tried to seize control of several neighbourhoods.
'Pools of blood'
Syria has denied accusations by special envoy Kofi Annan that state forces used heavy weapons or helicopters in clashes in the village of Tremseh last week, where activists said there was a massacre of over 100 people.
Jihad Makdissi, spokesman for Syria's Foreign Ministry, told reporters on Sunday that security forces killed 37 fighters and two civilians in a campaign against the village, from which the government said rebels were launching attacks on other areas.
"Government forces did not use planes, or helicopters, or tanks or artillery. The heaviest weapon used was an RPG [rocket-propelled grenade]," Makdissi told the reporters in Damascus.
"Yesterday we received a letter from Mr Kofi Annan addressed to the Foreign Minister Walid al-Moualem. The least that can be said about this letter about what happened in Tremseh is that it did not rely on facts. As diplomatically as possible, we say that this letter was very rushed.
A freelance photojournalist who visited Tremseh after Thursday's killings told Al Jazeera that he "found obvious proof of heavy weapons".
"Everyone was very nervous trying to show us what happened in the town. There were marks of shellings [by] mortars," said Daniel Leal Olivas
A UN Supervision Mission in Syria (UNSMIS) entered Tremseh on Sunday for a second day to assess the casualties and damage in the village.
The group released a statement on its inspections, saying that the village had come under attack by "direct and indirect weapons, including artillery, mortars and small arms".
"The integrated patrol, comprised of specialized civilian and military experts, observed over 50 houses that were burned and/or destroyed. Pools of blood and brain matter were observed in a number of homes," said statement, which did not identify the perpetrators of Thursday's attack or estimate the number of dead.
Opposition activists have said between 100 and 200 rebels and civilians were killed.
The International Committee of the Red Cross says it now considers the conflict in Syria a civil war, meaning international humanitarian law applies throughout the country.
The Geneva-based group's assessment is an important reference that helps parties in a conflict determine how much and what type of force they can or cannot use.