SINGAPORE -- Singapore doctors said on Tuesday they were debating whether to push on with a risky operation to separate Iranian sisters joined at the head for 28 years, saying more expert opinion was needed.

Experts say an operation on adult twins is unprecedented. Twins fused at the head occur once in every two million live births, and successful separation is even rarer.

Singapore doctors performed the operation in 2001 on infant girls from Nepal.

Iranians Laleh and Ladan Bijani have been put through a battery of tests since arriving in Singapore on November 20 in hopes of eventually living individual lives.

A medical team at Raffles Hospital in the city state said an independent ethics committee was evaluating plans for surgery on the twins, who were turned away by German doctors in 1996 after tests showed separating them could be fatal. "There are lots of ethical issues involved," Raffles Chief Executive Loo Choon Yong told reporters. "An independent ethics committee must study it and see whether there are any impediments, any ethical obstacles."

He did not elaborate on the ethical issues, Reuters reported.

The petite sisters, who have separately functioning brains encased in one skull, were "very keen" to have the operation, Loo said.

The twins would be left with a one-and-a-half palm-size cavity at the side of their heads if surgery went ahead, and risks death or permanent brain damage.

The sisters, both law graduates, have said differences between them spurred them to think about separation five years ago. They occasionally suffer from severe headaches.

Doctors are also grappling with issues of legal consent and arrangements for the twins' post-operative care. The operation is expected to cost between S$500,000 (U.S.$286,400) and S$600,000, but doctors are donating their services.

Loo said a decision on whether to conduct surgery could come in one to two months.

In April 2001, doctors at Singapore general hospital disentangled the fused skulls and intertwined brains of 11-month-old Nepali girls Jamuna and Ganga Shrestha in a four-day operation.

The lead surgeon in that operation, Dr. Keith Goh, is also head of the team examining the Bijani twins.