A giant panda loaned to Germany by China has given birth to tiny twin cubs, Berlin Zoo said Friday.

The as-yet-unnamed babies were born on Thursday afternoon, 11 days after an ultrasound detected two heartbeats in their mother, who was artificially inseminated in late March.

Pandas are notoriously difficult to breed because of the mother's very short reproductive window of 30 to 72 hours, typically once per year.

With just over 2,000 bears in the wild and captivity, breeding experts say every new cub born makes a difference to the future of the endangered species.

What do we know about the twins?

The Zoo said the cubs both weigh less than 170 grams (6 ounces) and are around 14 centimeters (5.5 inches) long.

As with other large bears, giant pandas are born almost naked, deaf, blind and pink and will only get their distinctive black and white fur markings later.

Their sex has not yet been determined "with certainty," the Zoo said.

The cubs are being cared for in an incubator donated by a Berlin hospital.

They will also alternate being with their mother every two to three hours to drink milk.

China-based panda breeding experts are in the German capital to offer support and advice.

Zoo warns of 'critical first few days'

"Both cubs are doing well and are being lovingly cared for around the clock by their mum and the experienced panda team. Now it's time to keep your fingers crossed for the critical first few days," the zoo said.

"I am relieved that the two were born healthy," zoo director Andreas Knieriem said. "The little ones make a lively impression and mom Meng Meng takes great care of her offspring." Knieriem said she was showing them plenty of "bear love."

The zoo said they would "actively support" Meng Meng as pandas usually raise one cub when they give birth to twins.

The twins and their mother won't be on show to the public for now, but their father, 14-year-old Jiao Qing — who arrived in Germany with Meng Meng — will be visible, as male pandas don't get involved in rearing cubs.

Second twins born to Meng Meng

The cubs are the second twins born to Meng Meng, a giant panda who arrived in Germany in 2017 as part of a loan agreement with China.

Her first twins, males Pit and Paule, were born in August 2019, the first to be delivered in Germany.

The twins were a star attraction in Berlin, but they were flown to China in December under the terms of the loan deal that were delayed by the COVID-19 pandemic.

China has gifted friendly nations with its unofficial mascot for decades as part of a "panda diplomacy" policy.

The country now makes commercial deals to loan pandas to zoos.

mm/rc (AP, DPA)