Forces loyal to President Bashar al-Assad "massacred" about 100 people, including many women and children, at a village in central Syria on Wednesday, the opposition Syrian National Council told AFP.
"We have 100 deaths in the village of Al-Kubeir" in Syria's central Hama province, "among them 20 women and 20 children," said Mohammed Sermini, spokesman for the exiled opposition coalition who accused the regime of being behind the "massacre."
Other sources also reported that a "massacre" had taken place in the same area, including opposition activists and the Syrian Observatory for Human Rights, which tentatively put the number of dead at 87.
Pro-regime shabiha militia armed with guns and knives carried out the "new massacre" at a farm after shelling by regular troops, the Britain-based Observatory said in a statement.
"What is certain is that dozens of people died, including women and children," the watchdog's Rami Abdel Rahman told AFP.
Both Sermini and the Observatory urged UN observers to immediately head to the region to investigate the latest atrocity in Syria's 15-month uprising against Assad's embattled regime.
At least 108 people were killed in a two-day massacre that began on May 25 near the central town of Houla, most of them women and children who were summarily executed, according to the United Nations.
The reports came after Russia and China said they were "decisively against" intervention or regime change in Syria, as Arab and Western calls mounted for strong international action in the conflict.
The United States endorsed an Arab proposal to invoke the UN Charter's tough Chapter VII, while refraining from supporting its powers to initiate military intervention.
Meanwhile, US Secretary of State Hillary Clinton sought to mobilise support in Turkey.
Clinton, who has voiced mounting frustration with the Chinese and Russian positions, called on the international community to "close off the regime's economic life lines."
"The regime must end the atrocities, comply with all its commitments under the Annan plan and allow the transition to a democratic Syria to begin," she said.
Clinton's Russian counterpart Sergei Lavrov warned regime change in Syria would lead the Middle East to "catastrophe."
Beijing and Moscow said after two days of talks between Russian President Vladimir Putin and Chinese leaders that they strongly opposed intervention and regime change.
"Russia and China are decisively against attempts to regulate the Syrian crisis with outside military intervention, as well as imposing... a policy of regime change," a joint statement said.
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Speaking in the Chinese capital, Lavrov urged the international community to resist calls from the exiled opposition to help oust Assad's regime.
Opposition groups "outside Syria appeal to the world community more and more to bomb the Assad regime, to change this regime. This is very risky, I would even say it is a way that will bring the region to catastrophe."
Lavrov hit out at the rebel Free Syrian Army's announcement it was no longer bound by UN-Arab League envoy Kofi Annan's six-point peace plan, and proposed a high-level conference with the participation of Iran among other powers.
Russia and China have vetoed two Security Council resolutions against Assad's regime, but backed Annan's blueprint to end the conflict in which more than 13,500 people have died since March 2011, according to the Observatory.
The Annan plan was supposed to begin with a ceasefire from April 12 but as violence has raged on daily despite the deployment of nearly 300 UN observers, doubts have emerged about its effectiveness.
In other violence, rebels went on the offensive in and around Damascus and 46 people were killed across the country, the Observatory said.
Rebels clashed with troops in Harasta and near Douma, Irbin amd Zamalka, all in the Damascus region, among other parts of the capital, according to the watchdog which says at least 168 soldiers have been killed in the past week.
According to the Britain-based watchdog's figures, at least 168 soldiers have been killed in the past week, including 76 at the weekend.
Ahead of a Security Council briefing by Annan on Thursday, Washington threw its support behind an Arab League proposal to invoke tough UN Chapter VII sanctions against Damascus -- without mentioning military intervention.
"We the United States hope that all responsible countries will soon join in taking appropriate actions against the Syrian regime, including, if necessary, Chapter 7 action in the UN Security Council, as called for by the Arab League last weekend," US Treasury Secretary Timothy Geithner said.
The United States was focused on boosting international economic sanctions against Syria "that can help hasten the day the Assad regime relinquishes power," Geithner said.
Assad appointed loyalist Riad Hijab as prime minister in a move France dismissed as a "masquerade."
Italy warned that Assad's policies risk creating "genocide" unless there is swift action to stop him.
Analysts said Syria risks descending into a long and bloody civil war with the Annan plan at a stalemate, the opposition badly fragmented and fierce resistance to any real changes by the Assad regime.
"I think you're going to find there'll be increasingly bloody episodes because the diplomacy has not managed to keep up with the situation on the ground," Salman Shaikh, director of the Brookings Doha Center, told AFP.