ALGIERS, Algeria >> A hulking military transport plane crashed just after take-off today in the worst aviation disaster in Algeria’s history, killing 257 people and plunging a nation where soldiers are especially esteemed into mourning.

An investigation was immediately ordered to determine the cause of the crash that killed soldiers, their family members and a group of 30 people returning from hospital stays in the capital to refugee camps in the south.

The huge plane, a Russian Il-76, crashed about 8 a.m. “just after leaving the tarmac” of the military airport in Boufarik, 20 miles south of Algiers, Maj. Gen. Boualem Madhi told the public TV station Canal Algerie. It crashed into a field just outside the base and was devoured by flames, killing 247 passengers and 10 crew members, the Defense Ministry said.

Video on the state-TV channel ENTV showed a blackened hulk broken into pieces, with huge wheels scattered about along with other plane parts. Firefighters doused the flames while body bags were placed in rows in the field.

President Abdelaziz Bouteflika ordered three days of mourning starting immediately and prayers for the dead on Friday at mosques across the country.

In the south, the Algerian-backed Polisario Front seeking independence for Western Sahara ordered a week of mourning for the 30 dead Sahrawi people from its refugee camps in Tindouf, a statement from the group said.

The flight was headed to Tindouf then Bechar, in the southwest, site of a military base, according to Farouk Achour, spokesman for Algeria’s civil protection services. Tindouf is home to many refugees from neighboring Western Sahara, a disputed territory annexed by Morocco.

There was no official mention of survivors, but one witness reported seeing some people jump out of the aircraft before it crashed. The Araban-language Algerian TV Dzair said five people were in a critical state but it was unclear if they had been inside the plane.

Several witnesses told Algerian TV network Ennahar they saw flames coming out of one of the planes’ engines just before it took off.

“The plane started to rise before falling,” an unidentified man lying on what seemed to be a hospital bed told Ennahar TV. “The plane crashed on its wing first and caught fire.”

The victims’ bodies were transported to the Algerian army’s central hospital outside the capital.

Algeria is vast and plane flights are often the best way to traverse Africa’s largest nation.

It was the first crash of an Algerian military plane since February 2014, when a U.S.-built C-130 Hercules turboprop slammed into a mountain in Algeria, killing at least 76 people and leaving just one survivor.

The four-engine Il-76 made its maiden voyage in 1997, according to Aviation Safety Network. The plane has been in production since the 1970s, and is widely used for both commercial freight and military transport. The Algerian military, which historically has depended on the Soviet Union then Russia for military hardware, operates several of the planes.

A retired officer, Mohamed Khelfaoui, told the online Algerian TSA site that he had flown in the aircraft several times and “it has proven itself in Algeria and elsewhere.”

It was not the worst crash of an Il-76. According to the Aviation Safety Network, the 2003 crash of an IL-76 of the Iranian Revolutionary Guard killed 275 people.

The heavy loss of life of soldiers was certain to deeply shake Algeria. The National Liberation Army — which grew out of the fighting force which freed Algeria from French colonial rule — is revered by Algerians.

Today, the army is credited with saving the nation from a deadly insurgency by Islamist extremists in the 1990s and early 2000s. The battle continues with sporadic attacks around Algeria and networks dismantled by soldiers.

The army’s experience fighting terrorism has made it a valued ally of the U.S. and other western nations.

The U.S. Embassy in Algiers expressed its “deepest condolences” to “our partners and colleagues in the Algerian military,” and their families, among the numerous condolences that flooded into the capital from far and wide.

The previous deadliest crash on Algerian soil occurred in 2003, when 102 people were killed after a civilian airliner crashed at the end of the runway in Tamanrasset, in the south. There was a single survivor in that crash.

Also in 2003, 10 people died when an Algerian Air Force C-130 crashed after an engine caught fire shortly after it took off from the airbase near Boufarik, according to the Aviation Safety Network.