Libreville - Gabon's president, Omar Bongo, has declared three days of mourning after a plane crash in the Gulf of Guinea which killed 16 people and has left three others missing, his office said on Wednesday.

"A national funeral will be organised after consulting the families of those lost," a statement from Bongo's office said, after the aircraft nosedived into the sea on Tuesday morning about 100m off Libreville's beach.

Eleven survivors - eight passengers and three of the four crew members - were treated in hospital in the capital, according to the official toll released after emergency services worked all day to save trapped people.

The survivors "were not seriously injured", a hospital official said late on Tuesday.

The 19 people dead or missing included two French nationals, a Lebanese and a German, their respective embassies said on Wednesday.

The turboprop Hawker Siddeley 748 passenger plane was headed from Libreville to Franceville in the southeast of the central African country when it crashed after takeoff, part of its landing gear falling off and landing on the beach.

Seven French nationals were aboard the plane - five passengers and two crew members - and five of them were among the survivors, the French embassy said.

The German embassy said that "a German citizen whose body has not been found" was on board the aircraft.

Two Lebanese people were also on aboard, according to Beirut's mission here. One survived and the other was known to have died.

When the plane, which belonged to private company Gabon Express, plunged into the water, its tail was jutting above the surface, raising hopes survivors could still be found.

Nearly four hours later the wreck was fully submerged with people presumed trapped inside.

Witnesses said they saw the twin-engined plane begin to head back to Leon Mba international airport before it crashed.

"The plane took off, then turned back with only one propeller turning," said a man who had just seen off a relative at the airport.

A woman on the beach who was waiting for returning fishing boats when the accident happened said: "Smoke was coming out of the plane. It plunged nose-first into the water, and people emerged."

The president's office said the accident happened "following technical problems", according to a statement read out on national radio.

The last of this type of aircraft, known as the BAe 748 after the company was merged into British Aerospace in 1977, was produced in 1988.