Gunfire was heard on Sunday in several neighbourhoods of Damascus, and protesters came out onto the streets and blocked roads to protest government offensives in those districts, the London-based opposition Syrian Observatory for Human Rights said on Sunday night.
The fighting in the Syrian capital is the heaviest since the conflict began at the beginning of last year, the group said.
It said ambulances were sent in to transport injured government forces away and killings were also reported among citizens but the numbers of the casualties were not immediately possible to verify, given the intensity of the fighting.
But the fighting on the streets is not the only concern for monitoring groups.
Donatella Rovera, an investigator with the rights group Amnesty International who recently spent several weeks in Syria, said it was clear that some opposition supporters and rebels had resorted to brutal tactics as they target members of the security forces, acts Al-Assad's regime has also been accused of, among others.
"They capture people, we've seen evidence of them having beaten them up … and in some cases they have killed them," Rovera said.
"Can it get much worse? It certainly can."
The strength of the armed opposition is growing, she said earlier in a speech at the Centre for Strategic and International Studies, and as clashes intensify, individual members are lashing out and committing human rights abuses by beating, detaining and killing Assad's soldiers.
Amnesty International is tracking these abuses in part by the YouTube videos members of the Free Syrian Army post online when they interview detainees.
Rovera stressed that the government was largely to blame for the escalating violence in Syria, saying Assad's forces have targeted whole villages in their attempt to suppress the spreading rebellion.
Marks of torture
In the latest reported massacre, the opposition said, government forces attacked the village of Tremseh in the rebellious Hama region with helicopters, artillery and tanks on Thursday.
Opposition sources put the death toll at anywhere from 100 to more than 200, although no independent account of the attack has surfaced.
Syrian state television blamed the deaths on "armed terrorist groups". Damascus has repeatedly said that it faces a foreign-backed insurgency waged by disparate groups including forces aligned with al-Qaeda.
Rovera said Amnesty International's reports out of Syria indicated that pro-Assad fighters had in some instances burned up to half of the homes and most clinics in towns to root out rebels, and are increasingly targeting unarmed civilians, including medical teams treating wounded rebels that have been barred from hospitals.
She said three medical aides were found dead in the city of Aleppo.
"The three were arrested and after a week, their bodies were found with clear marks of torture. Their nails had been pulled off, their teeth missing … and their bodies had been set on fire," Rovera said.
"It was to give a clear message that it is not a good idea to engage in these types of humanitarian tasks."
On the other side of the fence, Assad's security forces have been accused of engaging in torture techniques at 27 detention facilities by Human Rights Watch (HRW).
HRW, after speaking to victims of alleged violent abuse, identified locations, agencies responsible, torture methods used and the commanders in charge of the facilities.
The rights group's report "Torture Archipelago: Arbitrary Arrests, Torture and Enforced Disappearances in Syria's Underground Prisons since March 2011" is based on more than 200 interviews conducted since March 2011.
"The intelligence agencies are running an archipelago of torture centres scattered across the country," said Ole Solvang, HRW emergencies researcher. "By publishing their locations, describing the torture methods, and identifying those in charge we are putting those responsible on notice that they will have to answer for these horrific crimes."
Nearly all interviewees described a broad range of torture methods including prolonged beatings, holding detainees in painful stress positions, the use of electricity, burning with acid, sexual assault and humiliation, the pulling of fingernails and mock execution. Altogether HRW documented more than 20 distinct torture methods used by the security and intelligence services.
A 31-year-old detainee who was detained in Idlib governorate in June told HRW, "They forced me to undress. Then they started squeezing my fingers with pliers. They put staples in my fingers, chest and ears. I was only allowed to take them out if I spoke. The staples in the ears were the most painful. They used two wires hooked up to a car battery to give me electric shocks. They used electric stun-guns on my genitals twice. I thought I would never see my family again. They tortured me like this three times over three days."
HRW said that while most of the torture victims interviewed were young men between 18 and 35, the victims interviewed also included children, women, and the elderly.
The UN Security Council is expected to vote next week on extending the mandate of UN observers in Syria, whose original mission was to monitor a ceasefire that never took hold.
Rovera said it was important that the observers be allowed to stay on to help collect information on rights violations.
"The time for impunity is over," she said.
Violence in Syria has been escalating since protests erupted in March 2011. The unrest has killed more than 17 000 rebels, civilians and government forces, according to the Britain-based Syrian Observatory for Human Rights.
Meanwhile, the International Committee of the Red Cross (ICRC) now views fighting in Syria as an internal armed conflict – a civil war in layman's terms – crossing a threshold experts say can help lay the ground for future prosecutions for war crimes.
The ICRC is the guardian of the Geneva Conventions setting down the rules of war, and as such is considered a reference in qualifying when violence has evolved into an armed conflict.
The independent humanitarian agency had previously classed the violence in Syria as localised civil wars between government forces and armed opposition groups in three flashpoints – Homs, Hama and Idlib.
Hostilities have spread to other areas, leading the Swiss-based agency to conclude the fighting meets its threshold for an internal armed conflict and to inform the warring parties of its analysis and their obligations under law.
"There is a non-international armed conflict in Syria. Not every place is affected, but it is not only limited to those three areas, it has spread to several other areas," ICRC spokesperson Hicham Hassan said.
"That does not mean that all areas throughout the country are affected by hostilities," he said.
The qualification means that people who order or commit attacks on civilians including murder, torture and rape, or use disproportionate force against civilian areas, can be charged with war crimes in violation of international humanitarian law.
State of war
For most of the 17-month-old conflict, the ICRC has been the only international agency to deploy aid workers in Syria who deliver food, medical and other assistance across frontlines.
All fighters caught up in an internal armed conflict are obliged to respect international humanitarian law, also known as the law of armed conflict, according to the ICRC. This includes specific sections of the 1949 Geneva Conventions.
"What matters is that international humanitarian law applies wherever hostilities between government forces and opposition groups are taking place across the country [Syria]," Hassan said. "This includes, but is not necessarily limited to Homs, Idlib and Hama."
Andrew Clapham, director of the Geneva Academy of International Humanitarian Law and Human Rights, said the ICRC assessment of the conflict, which he shared, was important.
"It means it is more likely that indiscriminate attacks causing excessive civilian loss, injury or damage would be a war crime and could be prosecuted as such," Clapham said.
Al-Assad said in a speech on June 26 that his country was in a state of war.
The rules impose limits on how fighting may be conducted, so as to protect civilians and ex-combatants not taking part in the hostilities.
They require the humane treatment of all people in enemy hands and the duty to care for the wounded and sick. It also means parties to the internal conflict are entitled to attack military targets, but not civilians or civilian property. – Additional reporting by staff reporter