The UN observer force accused both sides in the Syrian conflict on Friday of hampering its peace mission and acknowledged its limitations in the face of escalating violence.
As the death toll from two days of violence topped 130, the Syrian Observatory for Human Rights, which tallies casualties, said UN observers should leave the country if they are unable to stop the killings.
The Britain-based group also issued an urgent call for the Red Cross and Red Crescent to send medics to the restive central city of Homs, where it said dozens of people have been wounded by government shelling.
France said Friday it was strongly concerned by reports it received of an impending, large-scale military operation by regime forces in Homs.
World powers could hold a summit on the crisis as the deadly anti-regime revolt entered its 16th month, while Washington said US and Russian leaders would work on Syria "disagreements."
"Violence, over the past 10 days, has been intensifying, again willingly by both the parties, with losses on both sides and significant risks to our observers," the UN force's chief Major General Robert Mood said in Damascus.
The unarmed observers have been targeted frequently since first deploying in mid-April to monitor a UN-backed truce. Earlier this month, Washington's UN envoy Susan Rice likened them to "sitting ducks in a shooting gallery."
Mood told a news conference the Syrian people were suffering the consequences of the failure to implement UN-Arab League envoy Kofi Annan's peace plan.
"There is no other plan on the table, yet it is not being implemented," the veteran Norwegian peacekeeper said. "Instead there is a push towards advancing military positions."
"This is not a static mission," he warned, adding its mandate would come under UN Security Council review at the end of July. "It is important that the parties give this mission a chance."
At least 36 people died in violence on Friday, including four demonstrators killed in the northern city of Aleppo by regime forces as thousands protested across Syria, the Observatory said.
A day earlier least 84 people died in clashes and bombings, 48 of them civilians, the Observatory said, bringing the overall toll from two days of violence to at least 130.
"The role of the international observers has become that of a witness to murder," the group said, adding the death toll had risen "dramatically over the last month."
"We call on the international observers to work for the immediate implementation of the Kofi Annan plan and ceasefire in order to stop the killings in Syria or to return to their home countries."
The watchdog also pleaded with the Red Cross and Red Crescent to dispatch urgent medical assistance to Homs immediately "to care for and evacuate dozens of injured hurt in the continuous shelling (by government forces) of several neighbourhoods, particularly Khalidiyeh."
Meanwhile, thousands of people took to the streets of Aleppo and other flashpoint areas.
Videos uploaded by activists showed anti-regime rallies outside mosques, in which demonstrators were heard calling for the ouster of President Bashar al-Assad and denouncing Russia's support for his government.
Among Friday's casualties were eight people killed in an explosion outside a mosque in Busra al-Sham in the southern province of Daraa, the Observatory said.
It also reported fighting between regime forces and rebels entrenched in the Homs province town of Rastan, and in the town of Andan in the northern province of Aleppo.
The State Department, meanwhile, announced that US President Barack Obama and his Russian counterpart Vladimir Putin will discuss differences over what to do about the conflict in Syria at a G20 summit next week.
"Obviously disagreements persist with regard to Syria, but it will be a good opportunity for the presidents to meet and work it through," State Department spokeswoman Victoria Nuland said.
Earlier Russian Foreign Minister Sergei Lavrov denied that Moscow has been discussing Assad's departure with Western nations, in comments seemingly aimed at quashing reports about a shift in its approach to Syria.
"There were no such discussions and there could not have been such discussions. This completely contradicts our position," he said.
"We are not involved in regime change," he said a day after Nuland talked of a "constructive conversation" with the Russians in Kabul on a transition plan modelled on Yemen.
French Foreign Minister Laurent Fabius, whose country along with the United States has been pushing for Assad to quit, said major powers could hold a conference soon in Geneva on the Syria crisis.
"There is a possibility of holding a conference in Geneva on June 30," Fabius told France Inter radio.
Participants would include UN Security Council countries, but the meeting would be held "without the constraints of the Security Council," he added.
Russia, along with China, has vetoed two Security Council resolutions against Assad and has vowed to oppose any military intervention.
In Istanbul, Syrian opposition leaders met in a bid to settle their differences and forge a united front to confront the escalating conflict. Arab and Western delegates and a representative of Annan attended the talks.
Human Rights Watch, meanwhile, accused Syrian government forces of having used sexual violence to torture men, women, girls and boys detained since the unrest began.
According to the Observatory more than 14,400 people have been killed since the uprising against Assad's regime erupted in mid-March 2011.