SAN'A, Yemen - Hundreds of Yemenia Airways employees protested on Tuesday to demand that their government be allowed to join a French investigation into the crash of a Yemeni airplane off the Comoros Islands two weeks ago.

The protesters presented a letter to the Yemeni government demanding an explanation over why the French are preventing Yemeni and Comoran rescue teams from being involved in the search operation.

Yemen says its requests to join French investigators have not been answered.

France said Monday it has been fully co-operating "with all the parties concerned," but officials could not be reached for comment Tuesday.

Flight 626 crashed June 30 on its way from San'a, Yemen, to Moroni, Comoros, killing all but one of the 153 people on board. The flight had originated in Paris.

Strong ocean currents are believed to have carried debris from the plane and some bodies over 500 miles (800 kilometres) northwest to the shores of Tanzania.

Yemen said investigators have found wreckage near the coasts of Tanzania and Kenya and said at least 20 dead bodies have been recovered. Yemenia officials say the bodies are in French custody and have not been handled by Yemeni authorities.

France announced Monday that it was sending a naval ship and underwater robots to help search for the plane's black boxes. Investigators have reportedly concluded that the black boxes - the plane's cockpit voice and flight data recorders - lie in waters too deep for divers, requiring the specialized robots to bring them up to the surface.

"We want to reach the black box and examine it with the most possible transparency in order to pin down the truth and what really happened in the last minutes before the crash," said Yemenia's chairman, Abdul-Khaliq al-Qadi.

Protesters in France have accused the airline of using good planes on European routes and worse ones on the leg from Yemen to the Comoros. Al-Qadi denied the allegations, saying his airline has a strong safety record.