African and EU leaders are holding a summit in Brussels where the conflict in the Central African Republic features prominently.
The UN Secretary-General Ban Ki-Moon promised to do “everything possible” to improve the international response to the crisis.
The European Union (EU) has already announced plans to send 1,000 troops to the country.
The UN has warned that 19,000 Muslims “face slaughter” in CAR.
Trade and immigration will also be high on the agenda at the summit.
The meeting is being attended by 30 heads of state and government – 15 each from Africa and Europe.
Zimbabwe’s President Robert Mugabe is boycotting the summit after the EU refused to temporarily lift a visa ban on his wife, Grace.
Mr Mugabe received support from South African President Jacob Zuma, who chose to boycott the summit in a show of solidarity for his Zimbabwean counterpart.
“I think that time must pass wherein we are looked [upon] as subjects,” Mr Zuma said. “We are told who must come, who must not come. It is wrong and causes this unnecessary unpleasantness.”
UN Secretary General Ban Ki-moon will attend the crisis meeting on CAR with EU and African leaders.
Mr Ban said the people of CAR were facing “grave and deplorable atrocities”, and he promised to do everything possible to improve the international response to the conflict.
The EU force was due to be deployed last month, but was delayed because of insufficient troop and aircraft commitments from the group’s 28 member-states.
The EU said its troops would operate in CAR’s capital, Bangui, which is worst-affected by the conflict.
“It is vital that there is a return to public order as soon as possible, so that the political transition process can be put back on track,” EU foreign policy chief Catherine Ashton said on Tuesday.
Some 8,000 African Union (AU) and French forces are battling to restore stability in CAR after more than a year of conflict, after mainly Muslim rebels seized power.
On Tuesday, the UNHCR said it was trying to evacuate some 19,000 Muslims from areas near Bangui.
Nearly 16,000 people had fled their homes in Bangui in the past 10 days amid an upsurge of violence, it added.
After rebel leader Michel Djotodia, who had become president, was forced to step down in January, vigilante groups began targeting Muslims across the Christian majority country.
UN human rights chief Navi Pillay said during a recent visit to CAR, she had been struck by the complete lack of any structures to support law and order in the country.
“What really shocked me is, for instance, the statement that was pertinently made to me by the prime minister himself, where he said effectively there’s not a state here,” she said.
“So there’s nothing, there’s no coherent army, no police, no justice system, hardly any proper detention facilities. So if you don’t have police and judges, you don’t have rule of law,” she added.
Apart from the CAR, the two blocs are due to discuss trade and investment.
EU council president Herman Van Rompuy said he hoped the summit would mark a new stage in Europe’s relationship with Africa.
It was time for a “shift from development cooperation to a partnership of equals with trade and investment playing a key role”, he added.
The EU has traditionally been Africa’s biggest trading partner and is by far its largest donor.
However, its dominance is increasingly being challenged by China, now a major investor in Africa.
“The need for investment in Africa is so huge that the more investment coming, the better,” EU development commissioner Andris Piebalgs told AFP news agency.