DAMASCUS: More than 1,000 families were trapped Saturday in the city of Homs and being pounded by regime forces, a watchdog said, as the UN accused both sides in Syria's conflict of willingly escalating the violence.

With world powers at loggerheads over how to stem the bloodletting, Syrian ally Russia meanwhile urged that pressure be increased "on both the regime and the opposition (to) make them cease fighting" and get them talking peace.

Rami Abdel Rahman, head of the Britain-based Syrian Observatory for Human Rights, said the families were trapped in the Khalidiyeh, Jourat al-Shiah, Qarabees, old city and Qusour areas of Homs, an opposition stronghold in central Syria.

"They have no food and no medical equipment," Abdel Rahman told AFP.

In a statement, the Observatory sent out an "urgent call" to UN Secretary General Ban Ki-moon "and all those with a sense of humanity to intervene immediately, in order to put a stop to the continuous shelling."

The NGO also called for the "evacuation and protection of dozens of injured."

"More than 100 people are injured, many of them badly, and the lack of medical equipment means some of them will die," Abdel Rahman said, adding there was also a lack of medical staff.

Home to several Free Syrian Army bastions, Homs has been under intermittent attack by regime forces ever since the district of Baba Amr was relentlessly pounded for a month earlier this year, according to the Observatory, and reclaimed by the regime.

An escalation of violence over the past week has engulfed several areas of the country, including Homs, and France's foreign ministry said Friday it was deeply concerned at reports of "an imminent, large-scale operation" by regime forces in the city.

The Observatory reported another 18 people killed on Saturday, 13 of them in Damascus province, adding to a death toll countrywide of at least 138 over the previous two days.

The watchdog said Syrian troops shelled a rebel bastion in the Damascus northern suburb of Douma during the night, killing seven people, while also among the dead was a family of three whose house in Irbin town was hit by a shell, and a regime soldier who died in a blast.

Three other regime troops died in clashes in rebel bastion Rastan, in Homs province, while two rebel leaders were killed near a regime checkpoint in the southern province of Daraa, the Observatory said.

The UN observer mission's chief, Major General Robert Mood, said in Damascus on Friday that 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.

"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," Mood told a press conference.

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."

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.

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 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. -- AGENCIES