For an all too brief moment, conjoined Iranian twins Ladan and Laleh Bijani achieved their ambitions to live separate lives yesterday.

After an unprecedented two-day operation to separate their skulls, the 29-year-old sisters were finally apart.

But, after suffering from huge blood loss, first Laden died and then, 90 minutes later, so did Laleh.

"We will think about them in the best of times. At least we helped them achieve their dream of being separated," lead neurosurgeon Dr Keith Goh said in the Singapore Raffles Hospital.

"Everyone upstairs is crying," said a nurse, who was involved in the operation.

Doctors had warned the sisters that the operation could kill them. But they were determined to live their own lives, and on Sunday teams of neurosurgeons began the marathon operation.

"When we undertook this challenge, we knew the risk was great. We knew that one of the scenarios was that we may lose both of them. Ladan and Laleh knew it too," said the hospital's chairman, Dr Loo Choon Yong.

In Iran, television programmes were interrupted with the news.

The 54-hour operation involved separating Ladan and Laleh's brains, which had fused together during their lives.

"Over the past six months, everyone who came into contact with them was touched by their personalities and the kind of people they were," said Dr Goh.

The sisters were extremely stable right up until the final cuts were made, but began losing large amounts of blood after they were separated, he said.

Working in two groups, the surgeons gave each twin blood transfusions, but in the end they were unable to cope with the unusual blood flow patterns.

"I was concentrating very hard on Laleh at the time," said Dr Goh, recounting the moment when he knew the operation had gone wrong.

"I was very saddened when I looked over and saw them struggling, of course, at the same time, we were struggling too."

A crucial milestone appeared to have been passed on Tuesday when surgeons cut a finger-thick vein the twins shared.

But the surgery was temporarily stopped that night when Ladan's new vein, which had been grafted from her thigh, became congested.

The medical team considered calling off the rest of the operation and leaving the twins joined. They knew that to proceed would be "very, very risky," Dr Loo Choon Yong said.

"The team wanted to know once again what were the wishes of Ladan and Laleh," he said. "We were told that Ladan and Laleh's wishes were to be separated under all circumstances."

Although the sisters knew the operation could kill one or both of them, they decided to face those dangers after a lifetime of living conjoined and compromising on everything from when to wake up to what career to pursue.

It was the first time surgeons had tried to separate adult craniopagus twins - siblings who are born joined at the head.

The procedure has been performed successfully since 1952 on infants, whose brains can recover more easily.

The sisters had hoped to return to Iran and live together. Ladan had wanted to become a lawyer and Laleh a journalist, friends said.