Giant panda Yang Yang and her twin cubs, which were born on August 7, 2016, are seen in this still frame taken from surveillance camera footage dated August 15, 2016 and released on August 16, 2016, in a breeding box inside their enclosure at Schoenbrunn Zoo in Vienna, Austria. [Photo/Agencies]

VIENNA - Officials from the Schoenbrunn Zoo in the Austrian capital of Vienna confirmed the birth of twin panda cubs on Tuesday.

Last week, they believed only one cub had been born by mother Yang Yang, though sounds from the breeding box appeared to be made by more than one. Staff then were on the lookout for a second cub which was confirmed later by camera footage.

The zoo said in a press release that signs from the cubs are promising, with both having "little fat stomachs" and mother Yang Yang appears "very relaxed."

While the twins are not often visible as Yang Yang warm them between her paws, the zoo did note that they are growing more fluff each day. Signs of their well-known black and white marks are already evident.

Zoo officials said giant pandas often give birth to twins, though mothers usually only raise the stronger one of the two. Their survival rate is only about 50 percent.