A suspicious device found on an Air France flight from Mauritius to Paris that prompted an emergency landing was harmless and caused a "false alarm", the airline's chief executive said on Sunday.

Air France staff decided to land the Boeing 777 at the nearest airport, in the Kenyan port city of Mombasa, after a passenger found the object in the toilets late on Saturday and alerted crew, CEO Frederic Gagey said.

The suspicious object was made out of cardboard and paper and contained a timer, Gagey said, adding that it must have been planted during the flight.

"All the information available to us at the moment indicates that the object was not capable of creating an explosion or damaging a plane," Gagey told a news conference in the French capital. "It was a false alarm."

READ: Air France flight makes emergency landing in Kenya after bomb alert

Mombasa County Commissioner Nelson Marwa said passengers had departed on another plane sent by Air France to pick them up. An undisclosed number who were being questioned by authorities had stayed behind, but he did not say if they were under arrest.

"They are now airborne, but a few among them who we were interrogating have remained with us, until ongoing investigations are done. We are still interrogating them," Marwa told Reuters by phone.

"This is a collaborative investigation involving Kenya and Mauritius, and when we are through, the plane involved will also be released."

Air France said it planned to take legal action over the incident, without giving further details.

The 459 passengers and 14 crew on board flight AF463 were evacuated using the emergency slides after the plane landed at 2136 GMT.

The device was retrieved from the aircraft by explosives experts from the Navy and DCI (Directorate of Criminal Investigations), Police Inspector General Joseph Boinnet said earlier on his Twitter account.

Kenyan Interior Minister Joseph Nkaissery told reporters at Mombasa's Moi International Airport on Sunday that the authorities were in touch with Mauritius to find out how passengers had been screened.

Air France said in a statement it had "immediately decided to reinforce the security measures in Mauritius" after the incident, which follows three bomb alerts in the United States in the last few weeks.

Airports of Mauritius, the operator of the airport in Plaine Magnien, said Deputy Prime Minister Xavier-Luc Duval would hold a meeting on Monday morning with government agencies involved in security to discuss measures taken following the incident.

"As precautionary measures, Mauritian authorities have tightened security procedures at the airport," Airports of Mauritius added in a statement.

Kenya's Airports Authority said on its Twitter account that normal operations at the Mombasa airport had resumed.

Gagey said arrangements were being made to fly the passengers and crew back to Paris, adding that they would return late on Sunday or early Monday.