Heavy clashes have erupted in Damascus between opposition fighters and government forces as the Red Cross says the country is experiencing civil war.
Activists in the capital said the Syrian army had closed down the road to the airport and was trying to surround fighters in southern neighbourhoods in an attempt to crush unrest inside Damascus.
Speaking via Skype, opposition activist Samir al-Shami said the fighting Tadamon followed a night of running battles in the nearby Hajar al-Aswad neighbourhood.
"There is the sound of heavy gunfire and there is smoke rising from the area. There are already some wounded and residents are trying to flee the area," he said.
"There are also armoured vehicles heading towards the southern part of the neighbourhood."
The UK-based Syrian Observatory for Human Rights said it was the "most intense" fighting in the capital since the beginning of the 16-month uprising against the rule of President Bashar al Assad.
"The regular army fired mortar rounds into several suburbs" where rebels of the Free Syrian Army are entrenched, Rami Abdel Rahman, the head of the opposition group, said.
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 fighting comes after an explosion struck a bus carrying security forces in Damascus, wounding several people.
The powerful blast appeared to have been caused by an improvised explosive device that had been stuck onto the vehicle, witnesses have stated.
There were conflicting reports about the number of casualties, with activists claiming more than one member of the security forces was killed, but others said there were no dead, only several people wounded.
News of the explosion and the renewed fighting in Damascus followed confirmation by the Red Cross (ICRC) that it now considers the Syrian conflict to be a full-blown civil war, meaning international humanitarian law applies throughout the country.
Previously, the ICRC had restricted its assessment to the hotspots of Idlib, Homs and Hama, but the organisation has determined that the hostilities have spread beyond those areas.
The assessment could prove significant as the violence continues, as it can form the basis for war crimes prosecutions, especially if civilians are attacked or detained enemies are abused or killed.
The Red Cross statement comes as UN observers returned to a Syrian village to continue their investigations after confirming the community there was attacked with artillery, mortars and small arms.
Following a visit to Tremseh in Hama province on Saturday, monitors said the assault mainly targeted rebels and opposition activists but added they could not confirm the number of dead and injured.
Opposition activists claim up to 200 people were killed on Thursday in the attack by President Bashar al Assad's troops, backed by a pro-regime militia known as shabiha.
TheSyrian Observatory for Human Rights' Rami Abdel Rahma, said he believed it "might be the biggest massacre committed in Syria since the start of the revolution" in March 2011.
The UN Supervision Mission in Syria (UNSMIS) said heavy weapons were used and homes burned, but stopped short of specifying who carried out the violence.
It is believed there may have been a fierce fight between the army and a number of deserters, with villagers caught in the crossfire.
The Syrian army said earlier it did launch an air and ground assault on Tremseh - but claimed only "terrorists" died in the operation.
Furthermore, the Syria government has rejected claims that heavy weapons were used, but admitted security forces killed 37 fighters and two civilians in a campaign against the village, from which the regime claims 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)," said Jihad Makdissi, spokesman for Syria's foreign ministry.
The UN observers' journey to the village comes even though the UNSMIS remains officially suspended after monitors repeatedly came under fire.
The UN Security Council also continues to debate the future of their presence in Syria, spurred by the July 20 expiration of its mandate.
Across the border in Turkey, hospitals treat injured Syrians and the country's prime minister has added his voice to the chorus of international condemnation of the Assad government.
"These vicious massacres, these attempts at genocide, these inhuman savageries are nothing but the footsteps of a regime that is on its way out," Recep Tayyip Erdogan said.
"Sooner or later, these tyrants with blood on their hands will go and the people of Syria will in the end make them pay."
Elsewhere, amid calls on Russia to support the removal of Mr Assad from office, the Kremlin has said President Vladimir Putin will meet the UN-Arab League envoy for Syria, Kofi Annan, this week.