United Nation's aid chief Jan Egeland has warned that conflicts in Sudan's Darfur, Chad and Central African Republic are now "intimately linked".
The African Union peacekeepers are badly over-stretched
He said fighters are crossing borders to launch attacks and risking a "really dangerous regional crisis".
His comments in Geneva come as the African Union meets in Nigeria, to discuss help for the overwhelmed and ill-equipped African force in Darfur.
Sudan has said no to the United Nations putting troops into the region
A hybrid mission was proposed earlier this month after a meeting of the UN, the AU and Sudanese delegates, but Sudanese President Omar al-Bashir again rejected this on Monday in a televised address.
He says he will only accept African troops under AU leadership.
Ahead of this Abuja meeting, the UN says it expects the Sudanese president to give some responses to three outstanding issues concerning this mission.
Mr Bashir says the Darfur crisis is invented by Western media
The size of the force, the appointment of a high representative who would report to both the AU and the UN and the appointment of a force commander.
UN Secretary-General Kofi Annan says he hopes the AU will use this meeting to press ahead.
He told the UN Human Rights Council in Geneva in a statement read out by the UN human rights chief, Louise Arbour, that violations in Darfur need urgent world attention.
Meanwhile, Mr Egeland warns that the governments in Chad, CAR and Sudan are involved to some degree in aiding rebel forces in its neighbouring countries and risking a much wider regional war.
"This false belief that the 'enemy of my enemy is my friend', is a fallacy really," he said.
"What you are creating is long-term chaos for yourself and a lot of suffering."
He called for immediate action to boost the peacekeeping effort.
There are currently 7,000 AU troops in Darfur who have failed to halt the violence. Many have been unpaid for months.
There have also been calls for troops to deploy to protect civilians and refugees from attacks across the border in Chad. UN aid agencies are moving non-essential staff out of the country after attacks in the east.
Rebels have also clashed with the government and French troops in the CAR town of Birao which is close to the Chadian and Darfur borders.
Mr Egeland said last week that the number of people in "desperate need" of aid in Darfur had risen to 4 million, compared to 1 million two years ago.
The UN estimates that at least 200,000 people have died from the effects of the three-and-a-half year war. Mr Bashir says 9,000 have been killed in the fighting.