Investigators were probing Tuesday how a fighter jet crashed during elite NATO training exercises at an air force base in Spain, killing 11 military personnel and leaving others with serious burns.

Nine French and two Greek personnel died and about 20 people were injured after the two-seater F-16 crashed into parked aircraft at the Los Llanos base in southeastern Spain on Monday.

Speaking before some 70 French military personnel at the site of the accident, France's Defence Minister Jean-Yves Le Drian said the tragedy "leaves a permanent mark on our collective memory."

"The entire defence community is deeply wounded, as are all are all our fellow citizens," he added.

Earlier on Tuesday on the plane to Spain he told reporters that the French military personnel who were on the ground when the accident occurred were "clearly traumatised by what they went through, helplessly."

"It is a horrific and unbearable accident," he added.

The jet -- involved in an elite training programme -- crashed after losing power as it took off, the Spanish defence ministry said, damaging two Italian planes and three French jets.

Firefighters rushed to the scene and battled the flames as black smoke billowed from the wreckage.

A source close to the French defence ministry said the F-16 "deviated quite prominently from its route, by 90 degrees, and hit French planes which were getting ready to takeoff".

"The planes were full of kerosene and there were many people around" which explains why there was a high number of fatalities, the source added.

At least one of the French pilots who died was already inside the cockpit of his plane, according to the source.

Two Greek pilots on board and eight French officers on the ground were confirmed killed on Monday, and the Spanish defence ministry said a ninth French victim died in a hospital burns unit in Madrid on Tuesday.

The dead included one Frenchwoman, a captain, the French defence ministry said.

It was the highest death toll in a single day for the French armed forces since an ambush in Afghanistan in which 10 people died in 2008.

Nine French personnel and 12 Italians were injured, officials from both countries said.

Three of the injured French nationals had been moved to a Madrid hospital while two others could return to France on Wednesday, Spanish Defence Minister Pedro Morenes told reporters at the base.

A judge in the eastern Spanish city of Valencia was leading an investigation into the accident.

French prosecutors also launched inquiries and French accident investigators were at the base, officials said.

A technical commission was also probing the causes and was set to examine the wreckage, the plane's black box recorders and recordings of controllers' conversations, a defence ministry source said.

- 'Tragedy for NATO' -

The base, near the city of Albacete, hosts elite exercises run by NATO to train military personnel from 10 nations to carry out joint manoeuvres.

Flags flew at half-mast at Nancy-Ochey air base in eastern France, where seven of those killed were from. Both houses of the French parliament held a minute's silence.

French President Francois Hollande "expressed his deep respect for the commitment" of the airmen, who were preparing for air force missions to fight "against terrorist groups" in Iraq and the African Sahel region.

NATO Secretary General Jens Stoltenberg called it "a tragedy which affects the whole NATO family," he said in a statement.

The 10 NATO countries that take part in the programme are Belgium, Britain, Denmark, France, Germany, Greece, Italy, the Netherlands, Spain and the United States.

The plane that crashed was taking part in NATO's Tactical Leadership Programme, which seeks to improve multinational cooperation in air operations.

According to the French defence ministry's website, it is "the most renowned and most demanding" programme for fighter pilots.

The F-16, manufactured by US company Lockheed Martin, is the biggest-selling fighter plane in the world with more than 4,500 made for 28 countries.