Kofi Annan has warned that if the situation in Syria does not change the future will be "all out civil war."
Speaking to the UN assembly in New York the UN-Arab League peace envoy called on the international community to unite, warning that if action is not taken, then Syria will continue to be wracked by "brutal repression, sectarian violence and massacres.
"All Syrians will lose", he told the assembly.
UN observers in Syria were unable to confirm a reported massacre of up to 100 civilians outside Hama on Thursday, after monitors were shot at with small arms when they tried to enter the villages, according to UN chief Ban-Ki Moon.
Following reports of a massacre outside Hama, Annan spoke frankly, saying despite President Assad agreeing to the ceasefire, "it is clear that the six-point-peace plan is not being implemented" and "the hour demands much more".
He expressed his horror at the killing, calling on the international community to "come together and unite, saying: "We must act and speak with one voice, for the sake of the Syrian people who are living through this nightmare."
Annan stressed the importance of acting quickly before the situation become "more radicalised and polarised" saying that international co-operation must be taken to a "new level".
"We cannot allow mass killing to become part of everyday reality in Syria" he told the assembly.
Earlier Ban-Ki Moon described the attack as "shocking and sickening" and of "unspeakable barbarity" saying that Assad has lost "all legitimacy", reports Sky News.
General Robert Mood said UN observers are being stopped at army checkpoints "and in some cases turned back". However he said observers are continuing to try and enter the region reported to have been attacked.
People described as civilians are also said to be preventing monitors from entering an area near the west of Hama, the BBC reported. However Syrian state TV has said that UN observers have entered the village, according to the BBC.
Horrific reports of the attack on the farming villages of Qubair and Maarzaf, 12 miles north of Hama, emerged from opposition activists on Thursday.
Activists close to the scene described some of the alleged atrocities, including one of a three-month old baby burned to death and others of men being stabbed outside their homes.
Pro-government militia and security forces are being blamed for the deaths. However Syrian authorities have blamed terrorists.
US Secretary of State Hillary Clinton described the "regime-sponsored violence" as "unconscionable" adding "Assad has doubled down on his brutality and duplicity and Syria will not, cannot, be peaceful, stable or certainly democratic until Assad goes."
Echoing David Cameron, Clinton said "the time has come for the international community to unite around a plan for post-Assad Syria."
She called on Assad to "transfer power and depart Syria" and reiterated the importance of intensifying the sanctions pressure as part of Kofi Annan's six-point peace plan.
At a meeting of the UN Security Council in New York UN-Arab League envoy Annan is expected to propose a contact group to try and revive the ailing plan, which was meant to see a ceasefire between opposition groups and Assad's regime in April.
Cameron said earlier: "If these reports are true, it is yet another absolutely brutal and sickening attack.
He called on the international community to "do much more" to "isolate Syria, to isolate the regime, to put the pressure on and to demonstrate that the whole world wants to see a political transition from this illegitimate regime and to actually see one that can take care of its people."
The council would include international and regional powers, including Russia and China, which would then negotiate with the government in Damascus.
However Russia and China are continuing to resist attempts to undermine Assad's regime, and refuse to support military intervention. In a statement issued on Thursday, Russia condemned the violence, without attributing blame for the attacks.
Foreign ministry spokesman Alexander Lukashevich said: "There is no doubt that certain forces once again used vicious and despicable provocations to frustrate Kofi Annan's plan.
It follows Russia and China's joint statement on Wednesday, in which they said they are "decisively against attempts to regulate the Syrian crisis with outside military intervention, as well as imposing... a policy of regime change" reports news agency, AFP.
Meanwhile the exiled Syrian National Council called on the UN "to stop waiting out of hope that the Assad regime will abide by terms of the peace plan."
It added in a statement on its website: "It has become clear Assad is abusing the plan to kill more Syrians."
A statement from the government aired on Syrian state TV denied the claims, saying that what is reported is "completely false", reports Sky News.
"The reports by the media are contributing to spilling the blood of Syrians" it adds.
The massacre in the villages outside Hama follow some of the the worst bloodshed since the start of the UN peace plan. In May, an attack on Houla, in central Syria, resulted in the deaths of more than 90 people, including 32 children under the age of 10, according to United Nations observers.
Local activist Abu Yazan told the paper that the shelling killed 12 people, with 106 killed by pro-regime thugs called "shabiha".
Videos posted to an activist network appear to show the killings in Hama, but they have not been independently verified.