An Air France plane from Mauritius to Paris is pictured at the Moi International Airport in Mombasa, after a suspected bomb was found on board.

AFP/Paris

French police on Monday detained a couple who were passengers on an Air France flight which made an emergency landing after a fake bomb was found on board, a police source said.

The couple were taken into custody by border police on their return to France, a day after their flight from Mauritius to Paris was forced to make an emergency landing in Kenya.

A passenger alerted crew members to the item found inside a toilet cubicle on Sunday on board the Boeing 777, which was carrying 459 passengers and 14 crew.

The plane landed in Mombasa where passengers where evacuated using emergency slides before navy and police bomb experts were called in to examine the suspicious item.

The object, made up of cardboard, paper and a kitchen timer, was found to pose no danger to the aircraft or its passengers, Air France chief executive Frederic Gagey said on Sunday.

He said the "deduction" was that the item had been placed in a toilet cupboard by one of the passengers and said the bomb scare appeared to be the result of a "bad joke".

Several passengers were questioned in Mombasa before a plane was sent to return all those on board to France.

An Air France spokesman told AFP the airline had taken legal action after the discovery of the suspicious object.

The airline "filed a legal complaint against unknown persons for endangering the life of others," the spokesman said.

France is on high alert after jihadist attacks in Paris in November left 130 people dead, and is one of many countries taking extra security precautions.

Airlines are especially jittery after Islamic State jihadists who claimed the Paris attacks also said they were responsible for downing a Russian jet in Egypt in October after smuggling a bomb onto the plane, killing all 224 people on board.

Gagey said there had been three bomb scares on Air France planes in the US in the past 15 days.