Russia insisted that new action against its ally Syria would be premature, amid Western calls for increased pressure on the regime as the death toll rises.
UN under secretary general Herve Ladsous gave the UN Security Council a "sombre" account of both the latest killings in the eastern town of Assukar -- 13 people killed execution-style -- and of last week's massacre near the central town of Houla in which more than 100 people died.
The United States, France, Britain and Germany all came out of a UN Security Council meeting on the worsening crisis urging measures up to sanctions by the 15-nation body.
All governments are increasingly concerned as the Syria death toll piles up and UN-Arab League envoy Kofi Annan left Damascus this week with no apparent concessions from President Bashar al-Assad.
The UN observer mission chief in Syria, Major General Robert Mood, said he was "deeply disturbed" by the latest killings in Assukar, calling it an "appalling and inexcusable act."
"Thirteen bodies were discovered last night (Tuesday) in the area of Assukar, 50 kilometres (30 miles) east of Deir Ezzor," the veteran Norwegian peacekeeper said.
"All the bodies had their hands tied behind their backs and some appear to have been shot in the head from a short distance," he added.
The bodies were found days after UN observers counted the bodies of at least 108 civilians near Houla.
Among them were 49 children and 34 women. Some were blown to bits by artillery and tank fire but most were summarily executed, according to the UN High Commissioner for Human Rights.
US ambassador Susan Rice told reporters at the United Nations that prospects for a political solution in Syria were now almost non-existent, and that the Security Council must discuss new action.
"That pressure could include sanctions of the sort that have been alluded to and discussed, and we were among those that raised that possibility," she said.
Syrian allies China and Russia, which have both blocked previous attempts at the Security Council to condemn the government of Bashar al-Assad, joined other council members on Sunday in approving a statement condemning last week's artillery bombardment of civilians.
But Russia insisted that the weekend's rebuke went far enough.
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"We believe that a review now by the Security Council of any new measures on the situation would be premature," Deputy Foreign Minister Gennady Gatilov said.
"It is essential to give the (peace) plan of Kofi Annan time to work," because intervention could "only exacerbate the situation for both Syria and the region as a whole," he told Interfax news agency.
The United States warned Russia and other backers of Syria they were on the wrong side of history.
"I would simply say that it is our belief, and it's the belief that we express in these conversations, that supporting the Assad regime is placing oneself or one's nation on the wrong side of history," said White House spokesman Jay Carney.
US President Barack Obama discussed the need for an end to the violence in Syria in a video conference with the leaders of France, Germany and Italy.
The White House said the consultations with French President Francois Hollande, German Chancellor Angela Merkel and Italian Prime Minister Mario Monti represented a follow up to the G8 summit Obama hosted earlier this month.
The four leaders discussed developments in Syria and their "shared perspectives on the importance of ending the violence of the government against its own people and the urgency of achieving a political transition," a White House statement said
Many Western governments ordered out senior Syrian diplomats on Tuesday in an apparently coordinated protest at the Houla killings but Russia slammed the move as "counter-productive."
Syrian forces launched a new assault on Wednesday on the site of last week's massacre, forcing villagers to flee heavy shelling in fear of more carnage, a watchdog and the opposition said.
Machinegun fire was followed in the afternoon by shelling that targeted the village of Taldu, near Houla, said the Syrian Observatory for Human Rights.
"People are fleeing Taldu to other parts of Houla," the Britain-based Observatory's Rami Abdel Rahman told AFP. "They are very afraid."
The Syrian opposition called for UN observers to rush to the area to protect residents.
"People are calling in distress following the regime's brutal attack on their community, especially after the army's withdrawal of roadblocks, which usually signals the beginning of attacks," said the Syrian National Council, the main opposition coalition.
More than 13,000 people have been killed, mostly civilians, since an uprising erupted against Assad's regime in March 2011, according to a watchdog's figures.
Meanwhile diplomats said Wednesday that Syrian government is stopping an Arab deputy to UN-Arab League envoy Kofi Annan from entering the country.
Nasser al-Qudwa, a former Palestinian foreign minister who is in charge of contacts with Syrian groups opposed to President Bashar al-Assad, has had several applications to go to Damascus turned down since he was named in March, diplomats said.