A member of the Free Syrian Army stands near the "Al-Hosn" Crusaders Citadel on the outskirts of Homs
Exiled opposition groups tried to forge a common vision for a transition in Syria as they met in Cairo as the UN rights chief accused both the regime and opposition of "serious" violations.
UN High Commissioner for Human Rights Navi Pillay also charged that weapons supplied to the government and opposition were escalating the conflict, warning that "further militarisation" must be avoided.
She spoke as at least 30 people were reportedly killed in new bloodshed across Syria.
Meanwhile, members of Syria's opposition met behind closed doors in Cairo to chart a common vision after criticising a blueprint for transition agreed by the major powers at the weekend.
Arab League chief Nabil al-Arabi, who chaired the two-day meeting attended by around 250 opposition figures, urged the factions "not to waste this opportunity" and to "unite."
Arabi also stressed the need for "a pluralist democratic system that does not discriminate between Syrians."
Nasser al-Qudwa, deputy to UN-Arab League peace envoy Kofi Annan, echoed Arabi's call, telling the opposition to "unify your vision and your performance."
"This is not a choice, but a necessity if the opposition wants to gain the trust of its people in Syria," Qudwa told the meeting which was also attended by the foreign ministers of Turkey, Iraq and Kuwait.
Rebel fighters and activists based in Syria said they were boycotting the meeting, denouncing it as a "conspiracy" and criticising the agenda which they said rejects calls for military intervention.
The statement, signed by the rebel Free Syrian Army and "independent" activists, said the meeting served the interests of the Damascus regime's allies Russia and Iran.
It also slammed the meeting for "ignoring the question of buffer zones protected by the international community, humanitarian corridors, an air embargo and the arming of rebel fighters."
In New York, UN rights chief Pillay briefed the UN Security Council before telling reporters that the violence was being fuelled by arms supplies to both the government and opposition.
"Any further militarisation of the conflict must be avoided at all costs," she said.
While Pillay did not name countries, Russia and Iran are key suppliers to President Bashar al-Assad's government, and Gulf states, notably Qatar and Saudi Arabia, have given weapons to the increasingly well-armed opposition.
Pillay said the government and opposition were carrying out "serious" new rights violations including attacks on hospitals and renewed an appeal for the council to refer the conflict to the International Criminal Court.
And she told the council that the violence, now in its 16th month, was becoming "increasingly sectarian," while hundreds of people remain trapped in Deir Ezzor, in the east, and in the Old City district of Homs "because of the increasing use of heavy weaponry, shelling and ongoing armed clashes."
With the United Nations considering the future of its observers in Syria, Pillay said she told the council it must "support and strengthen" the UN Supervision Mission in Syria so that it can "effectively" monitor events.
On Monday, the army kept up its bombardment of rebel neighbourhoods of the central city of Homs as violence killed at least 30 people across the country, said the Syrian Observatory for Human Rights.
More than 16,500 people have been killed in violence since the uprising against Assad's rule broke out in March last year, according to the Britain-based watchdog.
An activist in Homs told AFP via Skype that many civilians remained trapped in the shelling of the Jurat al-Shiah, Khaldiyeh and Old City neighbourhoods of Syria's third-largest city.
"Many neighbourhoods of Homs are still under siege, and it is really hard for us to get food or medicines in," said Khaled al-Tellawy.
"Field doctors are amputating the limbs of the injured because they have no equipment to treat them with, and they can't be smuggled out."
In the north, Turkey said that 85 Syrian soldiers defected after fleeing across its border. Among them were a general and other ranking officials, Anatolia news agency cited officials as saying.
World powers meeting in Geneva at the weekend agreed a transition plan for Syria, in a compromise with Russia and China that was branded a failure by both the opposition and Syrian state media.
The plan did not make any explicit call for Assad to cede power, as urged by Western governments, after Russia and China insisted Syrians themselves must decide how the transition unfolds.
The opposition Syrian National Council said on Sunday that "no initiative can receive the Syrian people's backing unless it specifically demands the fall of Bashar al-Assad and his clique."
But US State Department spokeswoman Victoria Nuland said the plan contained a clause that members of a transitional governing body to run Syria had to be agreed "by mutual consent."
"So from our perspective... there is no way that Assad, his cronies or anybody with blood on their hands is going to meet the mutual consent standard," she told reporters.
"This express, written assurance gives them ironclad guarantees that they will be able to veto people like Assad."
NATO chief Anders Fogh Rasmussen called on the international community to enforce a political solution in Syria while reiterating there would be no military intervention.