New Delhi: Doctors at the All India Institute of Medical Sciences on Thursday separated conjoined twins fused along their heads in a 16-hour surgery but refrained from calling the operation a success as the babies are still under observation in critical condition.

Twenty surgeons and 10 anaesthesiologists worked in shifts on the task of sundering Jagga and Balia, two-and-a-half-year-old boys from Kandhamal, Odisha, who were joined at the back of their heads, shared a key vein, and had what doctors call an "interdigitated brain".

"It is not over - this is the truth, the operation (separation) was only one phase. We'll have to observe them for several weeks and such separations have a very low survival rate," neurosurgeon Shashank Sharad Kale, a member of the surgical team, said.

The surgical procedure to separate the twins started with anaesthesiologists administering drugs to the babies at 6am and neurosurgeons began incisions at about 9.30am on October 25. The full separation was done at about 8.45pm, and surgeons spent the next four or five hours completing the surgery.

"Both babies are stable but will remain under observation in the intensive care unit," said Ashok K. Mahapatra, head of neurosurgery at the AIIMS who led the team.

The separation was preceded by an operation on August 28 in which surgeons grafted a vein into one baby to make up for the shared vein that he would lose after the separation.

Doctors say the twins' shared circulation had amplified the challenges related to the separation. During several weeks of observations and tests, including magnetic resonance imaging scans of the twins' heads, prior to surgery, doctors also improved the boys' general health through nutrition and medications.

The twins' shared circulation meant that specific medications intended for one child went into the other, raising the risk of complications ahead of and after the surgery, doctors said.

"One child has a complication relating to the heart and the other has a complication related to the kidney. We're just waiting and watching, we're optimistic," a member of the surgical team said on Thursday.

The interdigitated brain - a condition in which small sections of the brains of the two babies were interlocked - posed another significant challenge to the surgeons attempting the separation.

Craniopagus twins, joined at the heads, are rare and make up just one in 2.5 million live births. Although the AIIMS had separated conjoined twins fused along the abdomen earlier, this was its first attempt to separate craniopagus twins.

Doctors say two pairs of craniopagus twins are currently living in Hyderabad and Patna without separation.