ADDIS ABABA, Ethiopia -- African, Arab, European and U.N. leaders agreed in principle Thursday to a joint African Union-United Nations peacekeeping force for Sudan's Darfur region.

Sudan, which has strongly opposed allowing U.N. troops in the country, did not give the plan its unreserved approval because officials needed to consult with their superiors, said the country's U.N. ambassador, Abdul Mahmoud Abdelhaleem.

The force could be as large as 27,000, including the existing 7,000-member AU peacekeeping force in Darfur, but the leaders did not lay out a timetable for the force to begin work partly because Sudan had some reservations.

U.N. Secretary-General Kofi Annan said the additional personnel could include as many as 17,000 soldiers and 3,000 police officers.

A timetable for the expanded force to begin work was not announced partly because Sudan retained some reservations, including the question of who would be in charge.

"The next step is for the U.N. and AU to call a meeting of the non-signatories (of the Darfur Peace Agreement) ... and the government of Sudan. It should take place in the next couple of weeks to resolve outstanding issues by the end of the year," Annan told reporters.

The U.N. Security Council voted in August to replace the African Union's 7,000 troops, an underpowered force, with 20,000 U.N. peacekeepers. But Sudanese President Omar al-Bashir has so far refused to allow their deployment, saying they would be "neocolonialists."

The agreement was announced at a meeting in Ethiopia that brought together senior officials from the AU, the Arab League, the European Union, Sudan, the United States, China, Russia, Egypt, France and a half-dozen African countries.

"The United States welcomes the successful outcome of this historic meeting," Andrew Natsios, the U.S. special envoy to Sudan, said in a statement.

The expansion of the existing AU force will take place in three phases, said Annan, who had wanted to try to stop the bloodshed in Darfur before he leaves office on Jan. 1.

An African Union Peace and Security Council meeting will be held in the Republic of Congo on Nov. 24 during which Sudan is expected to present its final views, Annan said.

The senior British government representative at the meeting, International Development Secretary Hilary Benn, called on the Sudanese government to "accept the clear view of all the others present."

Benn called the joint U.N.-AU focus on Sudan "the best opportunity we have to bring this crisis to an end. In the mean time, we need an effective cease-fire, with all the parties committing to stop the fighting."

In recent days, pro-government militia forces known as janjaweed have stepped up attacks on villages in Darfur, killing dozens of people, international observers said Wednesday. In one raid, janjaweed militiamen -- backed by government troops -- forced children into a thatched hut, then set it ablaze, killing parents who tried to rescue the children, rebels said.

After years of low-level clashes over water and land in the vast, arid Darfur region, rebels from ethnic African tribes took up arms against Sudan's Arab-dominated central government in 2003. Khartoum is accused of unleashing the janjaweed. The militiamen are accused of many of the atrocities in a conflict that has killed some 200,000 people and chased 2.5 million from their homes.

The conflict has destabilized a wide region that includes parts of neighboring Chad and the Central African Republic. The chaos has been exploited by rebels from Sudan, Chad and the Central African Republic, and ethnic violence mirroring attacks in Darfur has been seen in Chad in recent weeks.

The Sudanese army has denied any connection to janjaweed attacks, saying the claims were politically motivated.

The U.N. humanitarian chief, who was visiting Darfur, said he had been told by people made homeless by the conflict that the withdrawal of non-governmental organizations from some areas had left them with fewer services and more exposure to violence.

"This is my fourth visit to Darfur, and I have never before seen such a bad security situation," Jan Egeland said from El Geneina, capital of West Darfur, according to a statement. "There are too many armed elements in and around the camps threatening the inhabitants and preventing us from going in."

"Aid workers in West Darfur cannot move on the roads because they are being attacked and their vehicles are being stolen," Egeland said.

Human Rights Watch has called for a major increase in the Darfur peacekeeping force to stop the growing number of attacks on civilians.

The New York-based advocacy group said it has documented renewed aerial bombing of civilians both in Darfur and inside neighboring Chad since late October.

The aid agency Doctors Without Borders, or Medecins Sans Frontieres, also reported that thousands of people have fled their homes and refugee camps in Darfur. The agency said it was increasingly difficult to provide aid to the victims because of the violence.