There is little evidence that Syria's government is complying with a peace plan to try to end violence, UN Secretary General Ban Ki-moon has said.
After briefing the Security Council, he warned there was an "imminent" danger of civil war in Syria and it could go from tipping point to breaking point.
Meanwhile, UN envoy Kofi Annan said the council should put more pressure on Damascus to implement his peace plan.
The comments come after reports that 78 people were killed in central Syria.
Opposition activists blame the killings at the Qubair village on pro-government forces but the government accuses "terrorists".
UN monitors trying to reach the village just hours later were fired upon, Mr Ban said. None were hurt in the shooting.
Mr Ban earlier gave several examples of how UN observers had narrowly escaped injury, saying heavy weapons rounds had fallen near a patrol and armour-piercing bullets had been fired at at least one vehicle, a diplomat told the BBC's Barbara Plett at the UN headquarters in New York.
The UN has 297 unarmed observers in Syria to verify the implementation of Mr Annan's six-point plan. It includes a ceasefire, which was supposed to have taken effect in mid-April.
Mr Ban was speaking at a joint news conference with Mr Annan and also Arab League Secretary General Nabil al-Arabi after a Security Council meeting on the Syrian crisis.
"The danger of a full scale war is imminent and real," the UN chief said, urging the Security Council to "speak with once voice".
Mr Ban also warned that recent violence across Syria "may amount to crimes against humanity".
"Reports of yet another massacre in Qubair underscore the horrifying reality on the ground.
"How many more times have we to condemn them, and how many ways must we say that we are outraged? The Syrian people are bleeding."
And while reiterating the UN was committed to Mr Annan's six-point peace plan remained "at the centre of our focus", he said urgent talks were needed to discuss how to proceed further.
His comments were echoed by Mr Annan, who warned that "further militarisation of the conflict would lead to disastrous consequences".
The latest discussions at the Security Council have highlighted divisions among its members, our correspondent reports.
She says that the US is demanding decisive action, while Russia and China are both opposing any outside intervention.
At the news conference, Mr Annan also confirmed that discussions were taking place on forming a contact group of key nations on Syria.
He said that such group should exert more pressure on both the government of President Bashar al-Assad and the opposition to comply with the peace plan.
It has become clear that the idea of including Iran in this contact group would be likely to lead to new controversy, correspondents say.
Although Mr Annan expressed hopes that Iran would be part of the solution, the US and UK earlier ruled out Tehran's participation.