By Niamh Kennedy, Stephanie Busari, Tim Lister, and Stephanie Halasz

(CNN) -- Chad's long-serving President Idriss Deby has died from injuries sustained in clashes with rebels, the army said in a surprise announcement Tuesday, plunging a volatile region deeper into crisis.

Army spokesman Azem Bermendao Agouna told state TV said Deby -- a longtime Western ally in efforts to quell Islamist militias in north Africa -- died "as a result of his injuries on the front line." Reuters said Deby had been visiting troops fighting rebels in the north of the country.

The announcement came a day after it was announced that Deby, who had been in power for 30 years, was projected to win a sixth consecutive term in elections held earlier this month.

Rebels, who have been seeking to oust Deby since 2016, had claimed a number of victories in the past week and clashes were reported in the north of the country at the weekend.

Rebels of the Front for Change and Concord in Chad -- known in French as Front Pour l'Alternance et La Concorde au Tchad (FACT) -- said Friday it had overrun a military garrison in Gouri. The claim was denied by the government, which instead said the rebels had been defeated.

On Monday FACT said that Deby was injured and on the run.

"Faithful to the oath made to the nation and the Chadian people, the Marshal of Chad, President of the Republic, Head of State, Supreme Commander of the Armed Forces, Idriss Déby Itno, has just breathed his last defending the sovereign nation on the battlefield," Agouna said in his statement. "It is with deep bitterness that we announce to the Chadian people the death this Tuesday, April 20, 2021, of Chadian Marshal Idriss Déby Itno as a result of his injuries on the frontline," he said.

Agouna said a transitional military council would take charge of the country for 18 months "to assure the defense of our dear country facing this war against terrorism and evil forces".

Deby's son, General Mahamat Kaka, will serve as president of the transitional council, according to Agouna. Analysts questioned whether the move was constitutional.

Agouna promised there would be "free, democratic and transparent elections following the spirit of sacrifice for which the marshal fought during his life."

The army declared a 14-day period of national mourning and imposed an overnight curfew. Air and land borders would close until further notice.

Chad worked closely with Nigeria and Cameroon in the fight against militant group Boko Haram and formed part of a joint taskforce fighting insurgents in the region.

Chad borders Libya, Darfur region of Sudan, Niger, Nigeria, Cameroon and the Central African Republic and there are fears Deby's death threatens the stability of the region. FACT claims it is in control of the Tibesti region of Chad.

Reuters reported that FACT, which is based across the northern border with Libya, had advanced hundreds of kilometres south towards the capital, N'Djamena, in the days after the April 11 election. But the Chadian military appeared to have slowed its advance about 300 km (185 miles) from N'Djamena, Reuters said.

The US Embassy in Chad directed US employees to "shelter in place" after to the death of Deby, citing the "potential for unrest." It urged US citizens to leave the country by commercial airline if possible. "Do not wait for the US Government to evacuate you. The US Government does not anticipate organizing any non-commercial evacuation flight from Chad," it said.

Deby, the 'security anchor'

Analysts said the situation was volatile. Mohammed Yahaya, the United Nations Development Programme (UNDP) Resident Representative in Nigeria told CNN: "The worst-case scenario is a Libya type of disintegration in what is a very insecure and conflict affected region. Chad under President Deby's leadership put a lot of pressure on Boko Haram in the region. If there's a disintegration would see an increased arms flow and an emboldened Boko Haram, that should concern any policymaker and security actors in Nigeria."

Yahaya said President Deby played a significant role for the international community as a "security anchor" in the region and their first thoughts would be how to secure an "orderly transition" in the country to avoid further instability.

"Deby was someone the international community relied on to bring security. He also contributed troops to Mali during the insurgency there, so my concern since I heard the news is 'what's next?' and how can the international community ensure and support the country through this difficult transition," he said.

But Deby's rule had become increasingly authoritarian. He had presided over the introduction of a a new constitution in 2018 that would have allowed him to stay in power until 2033, Reuters reported. although it did restore term limits. Deby took the title of Marshal last year and said before last week's election: "I know in advance that I will win, as I have done for the last 30 years," according to Reuters.

Matthew Page, an associate fellow on the Africa program at UK-based think tank Chatham House, doubted the constitutionality of the army's announcement of a military council headed by Deby's son. "Chad is not a hereditary monarchy, so it is obviously completely problematic because his son is not supposed to be his successor," Page said.

Page said France and the United States would be "extraordinarily nervous" about the latest developments. Both countries had supported Deby's "strongman approach" to running Chad, he said.

France -- the former colonial power in Chad -- said Tuesday that it had lost a "brave friend" and underlined the importance of a "peaceful transition."

"France stands alongside the people of Chad during this ordeal. It expresses its strong attachment to the stability and territorial integrity of Chad," the French president's office said in a statement.

"Chad has lost a great soldier and a President who has worked tirelessly for the security of the country and the stability of the region for over three decades."

The French military has about 5,000 troops battling Al Qaeda in the Islamic Maghreb and other terrorist groups in the area of northwest Africa known as the Sahel.