Rescue crews search waters near Navarre Bridge, east of Pensacola, Florida March 11, 2015. [Photo/Agencies]

Seven Marines and four soldiers were presumed dead after an Army Black Hawk helicopter crashed on a nighttime training mission off Florida's Gulf coast, where US military officials continued a search-and-rescue operation on Wednesday afternoon.

Some human remains had washed ashore, said a spokeswoman for Eglin Air Force Base in north Florida.

Officials did not immediately release information on what caused the crash involving the Marines and four members of the Louisiana National Guard. Heavy fog was reported around the time the helicopter was reported missing around 8:30 p.m. on Tuesday. Fog hampered the search effort on Wednesday.

A US military official, speaking on condition of anonymity, said the 11 service members aboard were presumed to have died in what could be among the deadliest domestic military training accidents in years.

One of two UH-60 Black Hawk helicopters participating in the routine exercise crashed near the base 50 miles (80 km) east of Pensacola, and rescue workers discovered debris about 2 a.m. on Wednesday, the base said in a statement.

The other helicopter "started to take off and then realized, I guess, that the weather was a condition and turned around," Major General Glenn Curtis of the Louisiana National Guard told reporters.

The second helicopter landed safely, the military said. Names of the missing troops were being withheld.

The Marines were part of a special operations unit from Camp Lejeune in North Carolina conducting amphibious "helicopter and boat insertion and extraction training" focused on getting troops in and out of specific areas, according to a Marine Corps statement.