Conjoined Iranian twins Ladan and Laleh Bijani died today from severe blood loss towards the end of a 50-hour pioneering operation to separate them.

Raffles Hospital in Singapore announced that the twins died as stage two of the operation - the neurosurgical separation of their conjoined brains - was "coming to an end".

A hospital statement said: "The twins lost a lot of blood and were in a critical situation. Doctors' attempts to stabilise them were in vain as their conditions continued to deteriorate. Despite the best efforts of the medical team, Ladan passed away at 2:30 pm. Laleh passed away shortly after, at 4:00 pm (0900BST)."

Dr Loo Choon Yong, chairman of the hospital, told of the medical team's agonising struggle to save the twins when their circulation began to fail after 50 hours of surgery.

He said the surgeons had, with "remarkable effort", successfully separated Ladan and Laleh before major complications with each twin's circulation arose.

He told a press conference: "(The team) continued to operate millimetre by millimetre right through the night. (Ladan and Laleh) tolerated the surgery well and the base of the skull had to be separated - thick bone - and that was successfully severed.

"The twins took 50 hours of anaesthetic and continuous surgery very well and we were hopeful but very cautious. Separation of the brain was achieved around 1.30pm. There was some bleeding but for a while they tolerated well.

"Surgery continued on the now separated Ladan and Laleh but around 2pm Ladan's circulation began to fail. Specialists battled to save her life but unfortunately she succumbed around 2.30pm.

"In the meantime Laleh was hanging on and surgery to her brain continued. However at around 3.25pm her circulation began to fail also. The team tried everything they could to save her but unfortunately we lost her. We were hoping and trying to do our best (against) the odds but alas we didn't make it."

Dr Yong said: "When we undertook this challenge, we knew the risks were great. But we were hopeful. Ladan and Lelah knew the risks too. As doctors there is only so much we can do as the rest we have to leave it to the Almighty."

Earlier, a nurse involved in the surgery told the Associated Press that "everyone upstairs is crying," after the death of the second twin.

The two women were joined at the skull, but had separate brains and were otherwise biologically distinct.

The team of doctors had to contend with unstable pressure levels inside the twins' brains just before they uncoupled the brains and cut through the last part of skull joining them.

The risky and lengthy procedure, which both women knew could kill them, began on Sunday morning.

Yesterday, five neurosurgeons completed one of the most dangerous steps in the surgery by rerouting a shared vein and successfully attaching a vein graft from Ladan's thigh.

The shared vein, thick as a finger, drained blood from the twins' brains to their hearts.

In 1996, German doctors had told the twins that the surgery was too dangerous, but the Singapore team had benefited from technological advances, according to hospital spokesman Dr Prem Kumar.

The operation was complicated further when the team discovered that the pressure in the twins' brains and circulatory system was fluctuating.

Dr Marc Mayberg, chairman of neurosurgery at the Cleveland clinic in Ohio, said that the pressure fluctuations could be fatal.

"If the pressure is due to the fact that there is insufficient drainage from this vein, in either cranium, that could be a life-threatening condition," he said.

After a lifetime of compromising on everything from when to wake up to what career to pursue, the sisters said that they would rather face a possibly fatal operation than continue living joined.

"If God wants us to live the rest of our lives as two separate, independent individuals, we will," Ladan said before the operation.

An international team of 28 doctors and about 100 medical assistants were enlisted for the surgery. The Iranian government yesterday said that it would pay the almost $300,000 (£200,000) cost of the operation and care.

This is the first time that surgeons have tried to separate adult craniopagus twins - siblings born joined at the head. Since 1952, the surgery has been performed successfully on infants, whose brains can more easily recover.

Participating neurosurgeon Dr Benjamin Carson, director of paediatric neurosurgery at Johns Hopkins children's centre in Baltimore, has separated three sets of craniopagus twins.

Because the operation on the Iranian twins was a medical first, surgeons encountered unexpected obstacles not seen in infants.

It took longer to cut through portions of their skulls because their older bones were denser, Dr Prem Kumar said.

The sisters were born into a poor family of 11 children in Firouzabad, southern Iran, but grew up in Tehran under doctors' care.

As girls, they used to cheat on tests by whispering answers to each other, they told reporters last month.

The government concluded that it would be almost impossible for them to compete individually in university entrance exams, so it granted them a scholarship to study law at Tehran University.

After surgery, the twins had hoped to move back to Iran and live together. Laleh wanted to pursue a career in journalism, while Ladan had hoped to work as a lawyer.