The humanitarian situation in eastern Democratic Republic of Congo has worsened drastically since fighting erupted in Bukavu earlier this month.
Thousands of people have fled fighting
The UN's emergency relief co-ordinator Jan Egeland said 3.3m civilians were out of reach of aid agencies - the largest number of any conflict.
Civilians were being targeted and the abuses are massive, he said.
"The world has not understood how deep the crisis has become and what is at stake," he said.
"There has been a marked deterioration since Bukavu fell and it has been spreading," he told Reuters news agency ahead of a UN Security Council debate on civilians in war zones later on Monday.
"Access wise, it is even worse than Darfur in western Sudan, where aid groups recently were permitted to enter."
The Congolese government has dismissed a threat by a dissident general to re-occupy the eastern town of Bukavu and instead urged him to surrender.
Foreign Minister Antoine Ghonda told the BBC that troops loyal to General Laurent Nkunda had committed crimes including looting and rape and should be brought to justice.
The rebel leader gave the government until Monday to look into alleged atrocities against his group.
Gen Nkunda told the BBC his troops could not allow new attacks on ethnic Tutsis by the government but did not say what action they would take.
The dissidents are Banyamulenge - as Congo's Tutsis are known.
The recent fighting in eastern DR Congo claimed about 90 lives and jeopardised the fragile peace process.
The renegade commanders are members of the Congolese Rally for Democracy (RCD) - a former rebel group backed by neighbouring Rwanda.
Their forces were supposed to be integrated into the DR Congo's new national army.
Last week, a UN spokesman in Bukavu said there had been some reprisal attacks against those seen as rebel sympathisers but by Thursday, the town was calm.
Some 6,000 Banyamulenge have fled to neighbouring Burundi.
Earlier, another rebel commander, Col Jules Mutebusi, told the BBC that his men had come under attack near Bukavu on Saturday.
He said his forces had no alternative but defend themselves.
The Banyamulenge have retained links to Rwanda's Tutsis - who dominate the army and government there.
Congolese President Joseph Kabila accused Rwanda of being behind the rebellion but this was denied by Kigali.
Gen Nkunda withdrew from Bukavu under pressure from the international community.
The UN has 10,800 troops in DR Congo - although those sent to Bukavu did not stop the town falling to the rebels.
According to a report by Human Rights Watch, government and dissident forces carried out war crimes during the last round of fighting, with both sides targeting civilians.
The dissidents in particular were accused of carrying out widespread sexual violence against women and girls as young as three years old.