A man writes in a book of condolences in Duesseldorf airport March 25, 2015.  [Photo/Agencies]

Germanwings said it cancelled one flight on Wednesday and was using 11 planes from other carriers for about 40 flights after some of its crew members had refused to fly.

Employees laid candles and flowers by Germanwings headquarters at Cologne/Bonn airport, while Lufthansa and Germanwings staff worldwide held a moment of silence at 10:53 am local (0953 GMT) - the moment the plane went missing.

Among the victims were 16 teenagers and two teachers from the Joseph-Koenig-Gymnasium high school in the town of Haltern am See in northwest Germany. They were on their way home after a Spanish exchange visit near Barcelona.

"On Tuesday last week we sent off 16 happy young people, with two happy young colleagues, on a journey and what was meant to be a happy trip ... has ended in tragedy," headmaster Ulrich Wessel told reporters.

Barcelona's Liceu opera house said two singers, Kazakhstan-born Oleg Bryjak and German Maria Radner, died while returning to Duesseldorf after performing in Wagner's "Siegfried".

Germanwings said on Tuesday the plane started descending a minute after reaching cruising height and lost altitude for eight minutes. Experts said that while the Airbus had descended rapidly, it did not seem to have simply fallen out of the sky.

The A320 is one of the world's most used passenger jets and has a good safety record.

At 24-years-old, the plane was older than many others at Lufthansa, where the average for its fleet is 11-and-a-half years. It was delivered to Lufthansa in 1991 and had clocked up around 58,000 flight hours over 46,000 flights, Airbus said.