Both 29-year-old Iranian sisters who had been joined at the head died Tuesday shortly after doctors in Singapore managed to separate them, the Associated Press reports.

Ladan Bijani died first, a spokesman for Raffles Hospital announced, and her sister, Lelah, died within 90 minutes, the wire service reports. The hospital says both twins died from uncontrolled blood loss and had still been under anesthesia.

The procedure was the first to be attempted on adult conjoined twins. Their brains touched inside their skulls, and although the organs weren't fused, they were nonetheless stuck together.

The doctors worked very slowly to avoid damaging any blood vessels and other tissue. The marathon procedure, which began late Saturday, had been expected to take about four days.

The neurosurgeons' work was being complicated by fluctuating pressure levels inside the twins' brains.

The surgery was led by Singapore neurosurgeon Dr. Keith Goh, assisted by six international experts and 18 local specialists, with a support team of about 100 medical staffers. The Iranian government had said it would pay the estimated $300,000 cost of the surgery.

The operation marked the first time surgeons had tried to separate adult craniopagus twins -- siblings born joined at the head -- since the procedure was first successfully performed in 1952. So far, the surgery has only been done successfully on infants, whose brains can recover more easily, the AP reports.