MELBOURNE, Australia (AP) -- A team of Australian surgeons were working Monday on a delicate and complicated surgery to separate twin sisters who are joined at the top of the head.

The 2-year-old Bangladeshi orphans, Trishna and Krishna, share parts of their skull, brain tissue and blood flow.

Doctors expected the operation, which began Monday morning, to take at least 16 hours, with a team of 16 surgeons and nurses.

"I am cautiously optimistic," plastic surgeon Tony Holmes said after the surgery began.

Holmes said the girls were sedated Sunday night and that an angiogram was performed to take a final look at the blood vessels before the operation. Doctors were first working to remove the bone at the back half of the girls' heads.

"It is a stressful time for any group of surgeons with this sort of case," Holmes told reporters. "They only come along really once in a lifetime and I think everybody has been on tenterhooks. We have had a few ups and downs with these children because of medical problems."

The girls were brought to Australia in 2007 by the Children First Foundation and have already had several operations in preparation for separation.

Doctors say the chance of a successful separation is 25 percent. There is a 50 percent chance the girls will suffer brain damage and a 25 percent chance one of the sisters will die.

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