By Colleen Jenkins and Letitia Stein

(Reuters) - 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 U.S. 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 U.S. 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.

"Our complete attention is on locating our fellow Marines," the statement said.

The four crew members and the helicopter were part of the Louisiana National Guard, assigned to an Army unit based in Hammond, Louisiana. All four were married men with children, Curtis said.

They were part of a highly experienced helicopter battalion, Curtis said, noting the two pilots were instructors and had "several thousand hours" of flying experience with their crew.

President Barack Obama phoned military officials to express condolences, White House spokesman Josh Earnest told reporters, noting the president anticipated a detailed investigation.

The helicopter that crashed had a flight data recorder that will be part of the investigation, a Louisiana guard official said.

The incident occurred near an Air Force base spanning nearly 500,000 acres (200,000 hectares) in the Florida Panhandle that is used extensively for training.

General Martin Dempsey, the top U.S. military officer, told a U.S. Senate hearing that the "loss of the folks on that helicopter" served as "a reminder to us that those who serve put themselves at risk both in training and in combat."

In February 2012, seven Marines were killed when two helicopters collided during a nighttime training exercise along the California-Arizona border.

The following year, another seven Marines died in an explosion at a Nevada munitions depot after a mortar round detonated prematurely during a live-fire training exercise.

(Additional reporting by Suzannah Gonzales, Phil Stewart, Curtis Skinner, Jonathan Kaminsky and Roberta Rampton; Editing by Peter Cooney and Will Dunham)