A fire has raced through a zoo in western Germany in the first few minutes of the new year, killing more than 30 animals, including apes, orangutans, monkeys, bats and birds.

Key points: Sky lanterns are both illegal and uncommon in Krefeld and most of Germany

Sky lanterns are both illegal and uncommon in Krefeld and most of Germany Police said used lanterns were found on the ground and several people were being interrogated

Police said used lanterns were found on the ground and several people were being interrogated Five orangutans, two gorillas, a chimpanzee and several monkeys were killed

Police said paper sky lanterns launched nearby to celebrate the arrival of 2020 were probably to blame.

Several witnesses saw cylindrical paper lanterns with little fires inside flying in the night sky shortly after midnight on Wednesday near the Krefeld Zoo, Gerd Hoppmann, the city's head of criminal police, told reporters.

"People reported seeing those sky lanterns flying at low altitude near the zoo and then it started burning," Mr Hoppmann said.

He said investigators also found used lanterns on the ground that hadn't burned entirely.

They were 34 centimetres long and made out of white paper, with an opening at the bottom where a small fire would be suspended. The fire heats the air inside, making them fly and shine at night.

Police and firefighters received the first emergency calls at 12:38am.

'An unfathomable tragedy'

People mourn outside the Krefeld zoo. ( Reuters: Thilo Schmuelgen )

The zoo, near the Dutch border, said its entire ape house burnt down and more than 30 animals — including five orangutans, two gorillas, a chimpanzee and several monkeys — were killed, as well as fruit bats and birds.

Only two chimpanzees were able to be rescued from the flames by firefighters. They suffered burns but were in stable condition, zoo director Wolfgang Dressen said.

"It's close to a miracle that Bally, a 40-year-old female chimpanzee, and Limbo, a younger male, survived this inferno," Mr Dressen said, adding that many of the zoo's animal handlers were in shock at the devastation.

"We have to seriously work through the mourning process," Mr Dressen said. "This is an unfathomable tragedy."

He said many of the dead animals were close to extinction in the wild.

The zoo said the Gorilla Garden, which is near its devastated Ape House, didn't go up in flames, and that gorilla Kidogo and six other members of his family were alive.

The zoo near the Dutch border said its entire ape house burned down and more than 30 animals were killed. ( AP: David Young/dpa )

Germans usually welcome in the new year with fireworks at midnight, and that people are allowed to buy and launch them.

Sky lanterns, however, are both illegal and uncommon in Krefeld and most of Germany. The mini hot-air balloons made of paper have been used in Asia for centuries.

After requests by police for witnesses to come forward, Krefeld police said several people had come in and were being interrogated. Police said they would not release details on them.

Mr Hoppmann said some of the partially burned lanterns had handwritten notes on them.

The Krefeld zoo, which opened in 1975, attracts 400,000 visitors each year. It closed after the fire and plans to remain closed Thursday.