Since rebel groups in the Darfur region of western Sudan took up arms against the government nearly two years ago, the violence has resulted in what the United Nations says is the worst humanitarian crisis in the world. From our East Africa Bureau in Nairobi, VOA correspondent Alisha Ryu examines the roots of the conflict and the enormous difficulties mediators face in trying to end the war.

The civil war in western Sudan erupted in February 2003, when an armed rebel group, the Sudan Liberation Army (SLA), began a series of attacks on government forces and installations in the region.

The Muslim Darfur-based rebels, made up of members of local tribes, say they launched the war to force an end to decades of political marginalization and economic neglect by the Arab-dominated government in Khartoum.

The SLA says it was also following the example of the southern-based Sudan People's Liberation Army (SPLA), which fought a devastating 20-year war with Khartoum for a similar cause, and was able to force the Sudanese government to begin negotiating for a power- and wealth-sharing peace deal.

After taking several key towns from government forces, the SLA linked up with another rebel group in Darfur, the Justice and Equality Movement, whose members come mostly from the powerful Zagawa tribe.

According to Sudan expert Richard Cornwell at the South African-based Institute of Security Studies, many Sudanese believe this rebel group formed as a result of a power struggle in 1999 between Sudanese President Omar Hassan al-Bashir and a former political ally, Hassan al-Turabi.

Mr. Turabi, who is a Zagawa, lost the power struggle, but remained a popular figure in Sudan. The United Nations has called the Darfur conflict the world's worst humanitarian disaster. Mr. Cornwell says it may have started, at least in part, as a power play.

"The Turabi link is very important," said Richard Cornwell. "I mean there are some people who are of the opinion that Turabi's supporters in Khartoum and Darfur deliberately manufactured this crisis with a view of taking power."

Since the beginning of the uprising in Darfur, the United Nations estimates that nearly 70,000 civilians have been killed and more than a million and a half others displaced.

The United States, United Nations and human rights groups believe rebel groups have committed their share of atrocities in the war. But most of the civilian suffering, they say, has been caused by violence perpetrated by the Sudanese government and its ally, the ethnic-Arab militia known as the Janjaweed.

An investigator with the human rights group Amnesty International, Benedict Goderiaux, describes the brutal way the Khartoum government has been responding to the rebellion in Darfur.

"What the civilians are subjected to is indiscriminate bombings by the Sudanese air force and more importantly, ground attacks by the Janjaweed, often accompanied by Sudanese army soldiers, who circle the village, kill people, beat women, sometimes rape women and girls, and then burn the homes, destroy the crops and loot the cattle," said Benedict Goderiaux.

The Sudanese government vehemently denies charges by the United States and the United Nations that it has been conducting ethnic cleansing, if not outright genocide, in Darfur. The government maintains that it is merely acting to defend against a rebellion that threatens national stability. Khartoum also denies charges it is supporting and arming the Janjaweed.

Observers say the government may have had little choice but to use the Janjaweed as a proxy army. They say many men in the Sudanese army come from Darfur would likely not have been enthusiastic about attacking people in their own region.

Analyst Richard Cornwell adds that Khartoum's decision to crush the Darfur rebellion could have also stemmed from a fear of having to negotiate another power and wealth sharing deal soon after making similar concessions to southern SPLA rebels.

"It could be that they're genuinely fearful that the concessions that they've made to the SPLA in the south, whether they intend to carry them out or not, has put them in jeopardy and they have to react in a stronger fashion," he said.

There is evidence that officials in Khartoum view the Darfur rebel movement as a serious threat to their 15 year hold on power.

The government has repeatedly accused the Darfur rebels, particularly the SLA, of having military ties with the southern rebels, and with neighboring Eritrea, which has long supported another rebel group on Sudan's eastern border.

Sudan says that both Eritrea and the SPLA have been secretly supplying money and arms to the Darfur rebels in a bid to form political and military alliances strong enough to overthrow the government in Khartoum.

Eritrea and the SPLA deny supporting the Darfur rebels. But in an interview with VOA last August, a senior military spokesman for the Justice and Equality Movement, Omar Adam, said that changing the leadership in Khartoum is a goal all the rebel groups in Sudan share.

"We believe that the best way to solve the Sudanese conflict, whether in the west or the south or in the east, is regime change because the Khartoum government is responsible for what's going on in Sudan," said Omar Adam.

Given the complex circumstances and open hostility between Khartoum and rebel groups, few observers are surprised that Darfur peace talks sponsored by the African Union seem to be making little, if any, progress. Indeed mediators are having trouble even keeping the talks going.