The Pentagon was partially evacuated on Friday morning due to heavy smoke near a construction area in the main concourse of the U.S. Department of Defense headquarters, officials said.

Investigators found heavy smoke but no fire in a mechanical room on the fifth floor. There were no injuries, the department said.

"It's all contained. It's under control, but they haven't issued an all-clear yet," the spokeswoman said.

Pentagon police and Arlington County, Virginia, firefighters were on scene investigating.

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The evacuation order for part of the Pentagon was still in effect and the building entrance near the building's Metro transit stop was closed.

The Pentagon is one of the world's largest office buildings, so big that the U.S. capitol building could fit inside any one of its five wedge-shaped sections. More than 20,000 people, both military and civilian, work inside.

A renovation has been under way for years, with construction currently focused on the main concourse, which houses shops, banking and mail facilities.