Ban Ki-moon, the United Nations secretary-general, has called on the UN Security Council take "decisive action" on the conflict in Syria a day after activists said scores were killed in a village in Hama province.
Ban warned on Friday that any failure to act would be giving "a license for further massacres" as he and Kofi Annan, the joint UN-Arab League envoy to Syria, called for more pressure on President Bashar al-Assad.
"I call upon all member states to take collective and decisive action to immediately and fully stop the tragedy unfolding in Syria," Ban said in a statement aimed at the council.
Meanwhile, Annan said he was "shocked and appalled" by the reports of the attack on the village of Tremseh in Hama province and condemned the government for using heavy weaponry in populated areas, the kind of aggression it was supposed to have stopped three months ago under a peace plan Annan drafted.
Opposition sources said rebels and civilians were killed when the village was attacked by helicopter gunships and tanks, then stormed by militiamen who killed entire families on Thursday.
The Syrian government said more than 50 people were killed when government forces clashed with "armed gangs" that were terrorising village residents.
The killings could not be independently verified since access for international media is severely restricted by authorities.
Air force operation
According to a report by the UN observer mission in Syria, a patrol of unarmed UN military observers could get within only about 6km of Tremseh on Thursday before being stopped by air force commanders because of "military operations".
In-depth coverage of escalating violence across Syria
The patrol observed the situation from a few different locations around Tremseh for about eight hours during which time
it heard more than 100 explosions, sporadic small arms and heavy machinegun fire and saw white and black smoke plumes.
The mission said Tremseh had been attacked as part of a continuing Syrian air force operation.
"The situation in Hama province continues to be highly volatile and unpredictable," said a "flash report" from the monitors obtained by Reuters.
"[The Syrian air force] continues to target populated urban areas north of Hama City in a large scale," the report said.
US Secretary of State Hillary Clinton expressed outrage over the killings in Tremseh and demanded that the Security Council take action to stop the violence.
"History will judge this council," she said. "Its members must ask themselves whether continuing to allow the Assad regime to commit unspeakable violence against its own people is the legacy they want to leave."
Russia, for its part, condemned the Tremseh killings, but echoed the Syrian government in blaming them on "terrorists" opposed to Assad.
Western nations have proposed a Security Council resolution that would impose sanctions on Assad for not ending the use of heavy weapons in the conflict, which the opposition say has left more than 17,000 people dead.
But Russia has rejected any use of sanctions, and proposed its own resolution that the West said fell short of expectations.