A team of Singapore doctors on Saturday surgically separated two 15-month-old conjoined Indonesian twin girls after more than 8 hours in the operating room.

But the operation on Angeli and Angie (search) — who were fused at the stomach and hips — was still continuing, said team leader Dr. Tan Kai Chah.

The surgery on the girls was expected to take up to 20 hours. Tan could not give further details on the success of the operation Saturday evening, as it was still underway.

Surgery is especially risky for Angeli, who has a hole in her heart, a condition that makes her chances of surviving lower than her sister's. But their parents said they continued to be optimistic.

"I am happy because the doctor has already separated their bodies. I hope that things will improve," said Neng Armaini, the twins' mother.

The girls arrived in Singapore in February, and have undergone tests to determine whether to go ahead with the operation.

Conjoined twins occur about once in every 150,000 to 200,000 live births.

Up to 60 percent of conjoined twins are stillborn, and 35 percent survive 24 hours or less. Those who survive longer are often plagued by medical complications due to shared organs (search) and vital systems.

Doctors in Singapore (search) — a wealthy, modern Southeast Asian city-state that promotes itself as a medical science hub — have had mixed results in separating conjoined twins in recent years.

In 2001, doctors separated the fused heads of Nepalese twins Ganga and Jamuna Shrestha (search) in an unprecedented 97-hour operation. The 5-year-olds now lie sick and virtually immobile in a cramped apartment in Nepal's capital.

Two years ago, doctors successfully separated a pair of South Korean twins fused at the spine, but weeks later, Iranian adult twins Ladan and Laleh Bijani — joined at the head — died from massive blood loss during an operation to separate them.