germanwings plane crash
Debris from an Airbus A320 is seen in the mountains, near Seyne-les-Alpes, March 24, 2015 in this still image taken from TV.Reuters

German airline Lufthansa is set to retire the flight number ― 9525 ― after the deadly crash of its low-cost carrier Germanwings flight 4U9525 on Tuesday that killed 150 people. 

The plane, which was flying with 144 passengers and six crew members from Barcelona, Spain, to Dusseldorf, Germany, crashed in the French Alps, making it the biggest air disaster for Lufthansa in more than two decades. 

Lufthansa has changed its logo on social media accounts to a black-and-white one from its otherwise bright yellow image as a sign of mourning. 

See Photos of Germwanwings Plane Crash

"Seeing the site of the accident was harrowing. We are in deep mourning. Our thoughts are with the relatives of the victims," Lufthansa CEO Carston Spohr said on Wednesday. 

"Germanwings and Lufthansa will do everything in our power to help in an uncomplicated and timely manner," he added. 

Meanwhile, Lufthansa is faced with an internal crisis, after many of its crew members refused to operate flights, following the disaster.

Several flights were cancelled after crew members cited concerns and fears, given that the 4U9525 flight crashed a day after it underwent hours of repair work. 

"One must not forget: many of our Germanwings crews have known crew members who were onboard the crashed plane. It is now more important to ensure psychological assistance if needed. And we will get back to a full flight operation as soon as possible then. But for me, this is rather secondary now," the Lufthansa CEO had said in response to the crews' stand.