An attack earlier this week on a western Syrian village that the opposition has described as a "massacre" mainly targeted army defectors and anti-government activists, according to United Nations observers who visited the scene.

The monitors, who arrived in Tremseh village on Saturday after earlier being denied access by the Syrian military, saw pools of blood and bullet cases inside some homes, as well as some destroyed buildings and a burned school, according to a statement released by Sausan Ghosheh, a spokeswoman for the UN mission in Syria.

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The attack "appeared targeted at specific groups and houses, mainly of army defectors and activists", she said, adding that a "wide range of weapons were used, including artillery, mortars and small arms".

Her statement did not identify the perpetrators of Thursday's attack or estimate the number of dead. Opposition activists have said between 100 and 200 rebels and civilians were killed. Ghosheh said the monitors hoped to return on Sunday to continue investigating.

The Tremseh attack has triggered a global outcry against the regime of President Bashar al-Assad, with UN chief Ban Ki-moon calling for "decisive" action to stop the bloodshed.

'Attempts at genocide'

The Turkish prime minister also condemned the intensifying violence, which the opposition says has killed more than 17,000 people since the uprising began in March last year.

"These vicious massacres, these attempts at genocide, these inhuman savageries are nothing but the footsteps of a regime that is on its way out,'' Recep Tayyip Erdogan said. "Sooner or later, these tyrants with blood on their hands will go and the people of Syria will in the end make them pay."

Activists said the village was surrounded by the military, attacked by helicopter gunships and tanks, then stormed by militiamen who killed entire families.

Emerging details of the attack suggest it may have been a lopsided assault on rebel holdouts and Tremseh residents who tried to protect their village. Nearly all of the dead were men, including dozens of armed rebels.

Residents said dozens of people remain unaccounted for, and that bodies remain in nearby fields that are inaccessible due to military checkpoints.

So far, it remains unknown why government forces launched a large offensive against Tremseh. The government said the killings were carried out by armed gangs and terrorists, its shorthand for the opposition. It said 50 people were killed before the army intervened and that three soldiers were killed.

The killings have added urgency to deadlocked Security Council negotiations on a Syria resolution.

The US ambassador to the UN, Susan Rice, said on Twitter that the killings "dramatically illustrate the need for binding measures on Syria" by the council.

Western nations have proposed a resolution that would impose sanctions on Assad's regime. Britain, France, the US, Germany and Portugal have proposed a resolution that would give the president 10 days to stop the use of heavy weapons, in line with the peace plan drafted by joint UN-Arab League envoy Kofi Annan, or face sanctions.

They also want to give the UN observer mission a new mandate, but for only 45 days. Their mandate ends on July 20.

Russia has rejected as unacceptable any use of sanctions. It is proposing a rival resolution that renews the mandate of the observers for 90 days.

Source: Al Jazeera and agencies