Fighting has escalated since the collapse of a short-lived October 2007 peace pact between the government and four rebel factions.

A February 2008 assault on the capital N'Djamena was the second rebel attack on the city in two years. The rebels, who call Deby's 18-year rule corrupt and dictatorial, warned the population to flee their homes, and thousands streamed across the river that marks the border with Cameroon.

The upsurge in violence delayed the deployment of a European Union force intended to protect aid operations in Chad and Central African Republic and contain any spillover from the conflict in Sudan's Darfur region.

Chad says Sudan is trying to overthrow Deby - whose health is bad - because it doesn't want the EU force to deploy on its border. Khartoum denies this and accuses Chad of aiding and abetting insurgents in Darfur, where around 2.5 million people have been uprooted by violence.

The fighting also threatens humanitarian operations supporting refugees and displaced civilians in Chad's east and south.

Columns of Chadian rebels again advanced into Chad from the border area in June 2008 and raided several towns.

Experts fear that the growing instability in Chad will lead to an all-out war with Sudan, giving Khartoum an excuse to crack down on Darfur rebels who are sympathetic to Deby.

INSURGENCY AND NEIGHBOURING TIES

A Chadian rebel rests in a camp on the Sudan-Chad border.

Opheera McDoom Deby seized power in a Libyan-backed coup in 1990 and went on to win the country's first two multi-party presidential elections in 1996 and in 2001, which critics say were rigged.

His opponents say Deby favours members of his own Zaghawa clan, who account for less then 3 percent of the population.

Deby changed the constitution in 2005 to allow him to stand for a third term, prompting a wave of desertions from the army.

In April 2006, government forces repelled an attack on the capital staged by the rebel group United Front for Change (FUC), under the leadership of Mahamat Nour Abdelkerim.

The following month, Deby held presidential elections, which were boycotted by opposition parties. Deby won, but his former supporters in the army continued to switch to rebel factions.

The largest rebel group is the United Force for Democracy and Development (UFDD), led by Mahamat Nouri. Nouri is a former member of Deby's government and was working as ambassador to Saudi Arabia when he defected two years ago. His forces are largely drawn from Nouri's Gorane ethnic group - the same clan as Chad's former President Hissene Habre, whom Deby ousted 17 years ago.

Deby's government has regularly accused Sudan of supporting Chadian rebel factions. Khartoum has repeatedly denied the allegations, but it has been widely reported that some groups have based themselves across the border and received equipment from Sudan.

Sudan in turn accuses Chad of backing rebels fighting the government across the border in Darfur - many of whom belong to the same ethnic group as the Chadian president.

Arabs of Chadian origin have joined Janjaweed militia - armed Arab groups accused of atrocities in Darfur - and are thought to exercise significant influence on Janjaweed cross-border raids into eastern Chad. Some Janjaweed attacks seem to be coordinated with those of Chadian rebels. Human Rights Watch says others are backed by Sudan with helicopter gunships and Sudanese soldiers.

The rebels' stated aim is to oust Deby, but local analysts believe some want to win concessions from the government regarding Chad's newfound oil wealth. Exploitation of the country's massive oil reserves began in the southern Doba region in June 2000.

Regional expert Alex De Waal says Chad's conflict is part of a regional competition for dominance through a vast arc of central Africa that includes Central African Republic and Democratic Republic of Congo. He says Sudan, Libya, Uganda, Congo, Rwanda and even Eritrea are vying for influence across this area.

EU INTERVENTION

A Chadian soldier patrols a pro-Deby rally in N'Djamena.

Claire Soares In response to the region's growing instability, in September 2007 the U.N. Security Council authorised a protection force for Chad and Central African Republic aimed at stemming the violence and preventing the possible spread of the Darfur conflict.

The planned deployment of up to 3,700 troops from EU countries and 300 U.N. police - which was delayed in the wake of the rebels' push into the capital in early 2008 - would be the European bloc's largest mission in Africa to date.

The force - EUFOR - has just over 3,100 troops in eastern Chad and northern Central African Republic.

The contingent is intended to complement a bigger U.N./African Union peacekeeping force planned for Darfur, where political and ethnic conflict triggered by a rebellion has killed at least 200,000 people since 2003, according to the United Nations.

Former colonial power France, which proposed the mission, has promised to provide roughly half the troops needed.

Although commanders have pledged to remain neutral, some analysts question whether the EU force will be able to avoid taking sides due to the involvement of the French military, which has assisted Deby under a bilateral defence accord with a small force of 1,200 troops and six Mirage fighter jets.

UFDD rebels have declared a "state of war" against all foreign forces - including the EU mission and France's separate contingent of soldiers and aircraft.

REFUGEES, AID AND HEALTH

Children walk around a camp for Darfur refugees in eastern Chad.

Jenny Iao U.N. agencies and international relief groups are providing aid to around 240,000 refugees from Darfur and 186,000 displaced Chadians in the east, close to the Sudanese border.

Another 45,000 or so refugees who have fled insurgency and lawlessness in northwest Central African Republic are sheltering in camps in southern Chad.

Most Sudanese refugees live in camps in the semi-desert east, and depend on the U.N. World Food Programme (WFP) for food. They compete with locals for scarce water supplies and firewood.

Displaced Chadians also face shortages of food, water, shelter and adequate sanitation. Their situation gets worse during the annual "hunger gap" before the September harvest. They have been attacked when trying to return home to plant crops or collect food.

Agencies delivering aid have to contend with extremely poor infrastructure, including roads that become impassable during rainy seasons.

Outbreaks of fighting, looting and armed attacks on aid convoys have regularly forced temporary suspensions of aid activities in eastern Chad. As well as banditry, battles between rebels and the army have boosted insecurity around the eastern towns of Abeche, Adre and Guereda, which are important hubs for aid workers helping hundreds of thousands of refugees there.

Armed Arab Janjaweed militias based in Darfur stage cross-border raids into Chad, attacking Sudanese refugees and Chadian villagers from the Masalit and Dajo ethnic groups.

Relief groups worry that escalating violence could threaten the entire international aid operation in Chad.

In February 2008, Chad's president Idriss Deby called on the international community to remove the refugees and threatened that the Chadian government would expel them itself.

Throughout Chad, access to health care is minimal. Around a fifth of children die before they reach the age of five, according to the U.N. World Health Organisation.

International aid agencies found their reputation knocked by a 2007 adoption scandal, in which a small French charity called Zoe's Ark was intercepted and accused of trying to kidnap 103 children from Chad. Zoe's Ark said it wanted to place orphaned Darfuri children in foster care with French families, but many of the children were found to be from Chad and had parents who were still alive.

The case caused a diplomatic wrangle with France, where a court sentenced six of the charity's workers to eight years in prison. And it sparked widespread anger in Chad, with hundreds staging protests in the eastern town of Abeche. Deby pardoned the six in March 2008.