UN Secretary General Kofi Annan has warned that Iraq is in danger of sliding into "full-scale civil war".
Iraqi civilians are killed and maimed on a daily basis
He called for urgent action from Iraqi leaders and the international community to bring Iraq back from the brink.
Mr Annan was addressing a meeting at UN headquarters in New York attended by Iraqi President Jalal Talabani and US Secretary of State Condoleezza Rice.
He spoke as world leaders arrived in New York for the UN General Assembly, which is due to convene within hours.
Monday saw more than 50 people die in Iraq in bomb attacks, shootings, and kidnap killings.
"Iraq and its leaders are now at an important crossroads," Mr Annan said.
"If they can address the needs and common interests of all Iraqis, the promise of peace and prosperity is still within reach.
"But if current patterns of alienation and violence persist much longer, there is a grave danger that the Iraqi state will break down, possibly in the midst of a full-scale civil war."
Mr Annan said Iraq had made "important progress" in the past two years, with national elections and a constitutional process.
But despite this, he said, it was "heartbreaking" that "the everyday life of Iraqi people is dominated by the constant threat of sectarian violence and civil strife".
Mr Annan has rarely been more forthright on the situation in Iraq than in these final weeks of his period as secretary general, says the BBC's Mike Wooldridge in New York.
Armed groups operate largely unchecked in Iraq
Last week, he said most leaders in the Middle East had told him on his recent visit to the region that the US-led invasion had been a disaster and had destabilised the region.
The communal violence in Iraq continued unabated on Monday.
A suicide bomber attacked volunteers who were queuing up at an Iraqi police recruitment centre in the city of Ramadi, killing 13 people.
Meanwhile, in the northern city of Talafar, at least 20 people died when a suicide bomber blew himself up in a busy market just before dark.
Fourteen bodies were found in different parts of Baghdad, showing signs of torture and bullet wounds to the head, the interior ministry said.
In and around Baquba, north of Baghdad, six people from two separate Shia families were shot dead as they packed up their homes, having repeatedly been ordered out of the area by armed gangs.