Unarmed United Nations observers looking to monitor Syria's conflict and the latest mass killing have been stifled by heavy weapons, armor-piercing bullets and surveillance drones, U.N. leader Ban Ki-moon said during a Security Council meeting. 

Ban told the U.N. General Assembly that the monitors "were shot at with small arms" as they tried to reach Mazraat al-Qubair, a farming area in the central Hama province where activists accused government forces of killing nearly 80 people, including women and children who were shot, hacked to death and burned in their homes. The group was denied access. By nightfall, the U.N. observers had not managed to visit the village, said spokesman Kieran Dwyer.

No observers were injured and it was not clear who was behind the shooting, the U.N. said.

Inside a closed Security Council meeting, diplomats told AFP that Ban said drones have been monitoring movements of the observers and heavy shelling was used to deter a convoy on a supervision mission.

U.N. officials say their vehicles are shot at almost every day in Syria, a tactic that Ban says is used to try to divert the observers from areas where the government has been accused of staging attacks.

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Ban added that the observers had seen Syrian military convoys approaching villages, but were "ignored" when they tried to prevent their assaults, AFP reports. 

The report of the mass killing in Mazraat al-Qubair came just weeks after more than 100 people were killed in one day in a cluster of villages known as Houla in central Homs province, many of them children and women gunned down in their homes. U.N. investigators blamed pro-government gunmen for at least some of the killings, but the Syrian regime denied responsibility and blamed rebels for the deaths.

The Houla massacre brought international outrage and a coordinated expulsion of Syrian diplomats from world capitals.

International envoy Kofi Annan, who tried to broker a plan to end the crisis, offered a grim assessment of the coming days and weeks in Syria and confirmed for the first time that Syria is not implementing his peace plan

"If things do not change, the future is likely to be one of brutal repression, massacres, sectarian violence, and even all-out civil war," Annan told the General Assembly. 

Annan blamed the failure of his peace plan primarily on the Syrian government Thursday and told the divided U.N. Security Council there must be "consequences" for those obstructing efforts to end the conflict.

"Clearly, the time has come to determine what more can be done to secure implementation of the plan," Annan said. "We must also chart a clearer course for a peaceful transition, if we are to help the government and opposition, as well as Syrian society, to help resolve the crisis."

U.S. Secretary of State Hillary Rodham Clinton condemned the Syrian government.

"The regime-sponsored violence that we witnessed again in Hama yesterday is simply unconscionable," she said in Turkey. "Assad has doubled down on his brutality and duplicity, and Syria will not, cannot be peaceful, stable or certainly democratic until Assad goes."

The exact death toll and circumstances of the killings overnight in Mazraat al-Qubair were impossible to confirm. The violence is bound to reinforce the growing belief that a peace plan brokered by Kofi Annan is unraveling as the country spirals toward civil war.

Both Homs and Hama have been centers of opposition to Assad's rule during the 15-month uprising.

The Britain-based Observatory for Human Rights said it had compiled the names of at least 49 people who had died in the massacre. But Rami Abdul-Rahman, director of the group which relies on a network of activists on the ground, said the circumstances of the killings were still unclear and called on U.N. observers to visit the area immediately.

The Local Coordination Committees, another activist group, group gave a higher death toll, saying more than 78 people were killed, including many women and children. It said pro-government militiamen known as shabiha first shelled the farming area and then went in and killed the residents there. It said some of the dead were stabbed to death while other bodies were burned.

A government statement published on the state-run news agency SANA said "an armed terrorsad is a member of the sect, while the opposition is dominated by Sunnis.

Attempts to reach more eyewitnesses and residents of the area was difficult, making the verification of what went on extremely difficult. The Syrian government keeps tight restrictions on journalists.

The statement claimed the killings were meant to put pressure on the Syrian regime ahead of a U.N. Security Council meeting.

British Prime Minister David Cameron urged concerted action from the international community against Syrian President Bashar Assad's regime following the latest reports. He said that if the reports of the "brutal and sickening attack" are true, it adds further proof that the Assad regime is "completely illegitimate and cannot stand."

Speaking during a visit to Norway, Cameron insisted more must be done to isolate Assad's regime and show that "the whole world" wants to see political transition in Syria and condemns "absolutely" the Syrian regime.

In Paris, French Foreign Ministry spokesman Bernard Valero said ministers from the so-called "Friends of Syria" countries -- many European and Arab nations -- would meet in the French capital on July 6 to help support the Annan plan. He said the meeting would mobilize "all states and organizations that want to support the Syrian people" amid the repression.

The Associated Press contributed to this report.