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The UN observer force accused both sides in the Syrian conflict of willingly escalating the violence, as a watchdog Saturday reported more than 150 killed in three days of fighting around rebel bastions.
The UN accusations came as Russia said international pressure should be increased "on both the regime and the opposition (to) make them cease fighting" and hold talks instead.
The Syrian Observatory for Human Rights said UN observers should leave the country if they cannot stop the bloodshed, while reporting another 18 people killed on Saturday, adding to a death toll of at least 138 over the previous two days.
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's foreign ministry said on Friday it was deeply concerned at reports of "an imminent, large-scale operation" by regime forces in Homs.
French Foreign Minister Laurent Fabius, meanwhile, said world powers could hold a summit on the crisis, 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 on Friday.
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 that 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."
On the ground, the Observatory said Syrian troops shelled a rebel bastion in northern Damascus overnight killing seven people, including three women.
It reported six other people killed in Damascus province, including a family of three when their house was hit by a shell and a regime soldier who died in an explosion.
The watchdog said three other regime troops died in clashes in rebel bastion Rastan, in the central province of Homs.
Elsewhere, two rebel leaders were killed near a regime checkpoint in the southern province of Daraa, it said.
"The role of the international observers has become that of a witness to murder," the Observatory said, adding that the death toll had risen "dramatically over the past 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 US State Department, meanwhile, announced that US President Barack Obama and his Russian counterpart Vladimir Putin will discuss differences over 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," spokeswoman Victoria Nuland said.
Russian Foreign Minister Sergei Lavrov, in an editorial in the online Huffington Post, said Moscow was working with Syrian authorities on an almost daily basis to urge them to implement the Annan plan and "resolutely abandon their delusion that the internal political crisis in Syria will somehow go away."
"We need to bring all the weight to bear on both the regime and the opposition and make them cease fighting and meet at the negotiating table," he said, promoting Moscow's call for an international conference on the crisis.
France's 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 Syria.
"There is a possibility of holding a conference in Geneva on June 30," he told France Inter radio.
Participants would include UN Security Council countries, but the meeting would be held "without the constraints of the Security Council," Fabius added.