Khalia Daoud squats in the shade of her straw hut with hundreds of other Chadian refugees, mostly women, children and the elderly. Her one-square-metre makeshift shelter stands at the edge of Sudan's porous 1300-kilometre border with Chad.

Donkeys mill about, and under a dried-up thorn tree some women display a meagre assortment of root vegetables, small piles of millet, peanuts and spices, in the encampment's only market.

Khalia, a mother of six, fled to Darfur more than a month ago but has received little help from aid agencies, which are reluctant to travel in an increasingly dangerous situation. Still, she is glad to be free of the harassment and attacks that occurred frequently in her village in Chad. "It seems as if we are welcome here. No one has harassed us yet," she says.

But the trouble from which she fled may have followed her. Operating from bases along the border inside Sudan, Chadian rebels have started staging attacks on their home country. Since their first big attack, on the eastern city of Adre in December, their numbers have grown to about 9000 well-armed men.

A similar number of refugees from Chad have arrived in western Sudan's Darfur region in the past couple of months. They have fled attacks from rival tribesmen backed by the ailing, beleaguered President of Chad, Idriss Deby, and the non-Arab tribe from which he hails, the Zaghawa.

The arrival of Chadian refugees is an alarming new development in Darfur, where more than 200,000 Sudanese have died, largely at the hands of government-backed Arab militias, and 2 million have endured famine as well as displacement. Until now the crisis in Darfur worked the other way around: Sudanese refugees being displaced within Darfur and flowing into Chad.

The proliferation of rebel groups, the rising tensions between Chad and Sudan, and the enmity between the rival tribes that live on both sides of the border are making things even worse for those caught in the middle.

The recent developments lay bare the underlying tribal complexities of the conflict in Darfur, which is looking more and more like Somalia, with tribal-based militias fighting each other more than the government, and a steady flow of arms on both sides of the border.

"In Chad the Zaghawa and Massalit tribes take anything from us they want and you can do nothing," says Ibrahim Osman, a 30-year-old sheik at one of the encampments along the border inside Darfur. "I hear that the problem here in Darfur is the same, only reversed."

The Zaghawa, Massalit and nearly every other tribe caught up in the conflict live on both sides of the border. Sheik Osman concedes that many of his camp's inhabitants are the wives and children of the Chadian rebels. Aid workers long suspected this and have feared their aid supplies might fall into rebels' hands if delivered to the Chadian refugees. But that is not the only reason aid workers rarely come around.

There have been more than 70 attacks on aid workers in West Darfur in the past four months. One international aid agency in West Darfur recently negotiated passage through Chadian rebel-controlled territory, something aid agencies are regularly forced to do with different rebel groups.

"We are working with a sovereign country … they should ensure our safety," says Andy Pendleton of the UN Office for the Co-ordination of Humanitarian Affairs in West Darfur, referring to the Sudanese Government. "We shouldn't have to deal with fighting forces that have commandeered a piece of the pie."

Darfur has long been used as a staging ground for attacks by Chadian opposition forces, and although a loosely organised armed opposition to President Deby has existed since 1994, its strength has only recently increased dramatically, with a number of prominent defections from the Chadian Army. Mr Deby has fallen out of favour with the outside world and his own people, each dissatisfied with rampant corruption and nepotism.

"All governments which govern Chad were either directly or indirectly installed by causes from Darfur, or were toppled or weakened by incidents happening in Darfur," says Dr Eltayeb Hag Ateya, an expert on Darfur at the University of Khartoum.

In a rare case of contact with the press, the Chadian rebels allow a viewing of a military parade. A punishing four-hour drive outside Geneina, their base is situated along the border, past two burnt-out villages and beside a confluence of dry riverbeds.

In the morning hundreds of Chadian rebels congregate for the parade. Wearing mismatched uniforms, they converge from nearby camps with Kalashnikovs slung over their shoulders and many sporting Rambo-style chain-link belts of large-calibre bullets. Forty new Toyota LandCruisers packed with young fighters, some just past adolescence, line the edge of the open, dust-choked field. Tied to the sides of the four-wheel-drives are sacks filled with rocket-propelled grenades.

Under the shade of a mango tree the leader of the Chadian rebels, Mohammad Nour, decries the corruption rampant in Chad and lays out a vision for a post-coup transition to democracy.

"I am not a politician and have no intention for being a politician," says the 35-year-old Mr Nour, who fought in Mr Deby's Darfur-based coup in 1990 before becoming fed up with him. "What we want to do for Chad is kick out President Deby - if he refuses to sit down with us and others."

After the ousting, Mr Nour envisions an Afghan-style loya jirga forum with traditional leaders from across Chad to determine the nature of a transitional government, which will in turn set out a timetable for democratic elections.

Past the idealism of sweeping reforms, the tribal overtones behind the Chadian rebels' politics are clear: inside the camp are members of various Arab militias, which have long been welcomed and supported by President Omar al-Bashir of Sudan and marginalised by President Deby in Chad.

Mr Nour's assessment of the Darfur crisis - which Western governments have labelled genocide and laid squarely at the door of Mr Bashir's government - is similar to explanations routinely offered by officials in Khartoum.

"There is no Sudanese rebellion. It is all a tribal problem," Mr Nour says dismissively. He insists the only support he is getting from Sudan is "free access."

His commanders finish their speeches with roars of self-satisfaction and conviction, and the soldiers start driving off. As their tyres throw up dust from the parched earth it is difficult to tell which direction along the unmarked border they are speeding off to.

Back at the refugee camp along the border, Khalia Daoud knows this all too well. "It's hard to tell the sides apart. All of the villages are mixed," she says. But her tribe is on the side of the rebels, so she and her children prepare to spend another night in a foreign country.