The building's walls are designed to withstand a missile strike and a highly secured operational room is hidden underground. France's army chiefs are moving into their new defense ministry, aimed at allowing a quicker response to the threats faced by the country — especially terrorism.

About 9,300 military and civil staff who were previously dispersed around a dozen different sites are now based in the 4.2 billion euros building ($4.6 billion), dubbed the "French Pentagon."

The joining together of army, air force and navy headquarters will make it easier to lead France's military operations abroad, said Jean-Paul Bodin, secretary general for the administration of the Defense Ministry. "This enables us to be in contact with each other much more easily than before and also to mobilize the staff quicker when needed."

France's military is highly active internationally, with about 7,000 French troops involved in operations around the world, including in the anti-Islamic State coalition in Iraq and Syria and in operations against extremists in Africa's Sahel region. An additional 7,000 troops have been mobilized to patrol sensitive sites across France, following attacks in Paris in January on satirical newspaper Charlie Hebdo and a kosher grocery that left 20 dead, including the three Islamic extremist attackers.

Bodin said the new military organization will make the army's decision-making process "more efficient."

It will also help reduce costs and staff in the headquarters, as part of a government plan to reduce France's military from 270,000 people to 240,000 by 2019, he said.

The ministry's employees left behind historical buildings in the center of Paris for the new modern site, in a quiet district in the south of the capital.

Yet Defense Minister Jean-Yves Le Drian keeps his office in a prestigious Paris mansion from the 18th century, closer to Parliament and the presidential palace.

The new ministry is to be inaugurated by president Francois Hollande next month. It is named the "Hexagone-Balard" — the hexagon being the shape used to describe France's mainland, and also the shape of the heart of the building.

From the outside, the seven-floor structure looks like a modern fortress with its white opaque glass front. It is topped by the largest solar-paneled roof of Paris.

Inside, it evokes a small university campus with its succession of gardens surrounded by blue and green facades, and a series of facilities including a hairdresser, library, swimming pool, sports rooms, restaurants and preschools for the staff's children.

Specific military equipment is being kept top secret — even from the architects.