JAKARTA, January 23 (RIA Novosti) - Indonesian doctors are planning to separate conjoined twin boys who were born a week ago, but said the "complex" surgery can only go ahead once the babies are strong enough, local media said on Friday.

"The boys named Asni I and Asni II were born a week ago, joined at the chest and belly. They are too small and weak for surgery, but as soon as their state allows it, we will separate them," the Antara news agency quoted doctor Azwan Lubis as saying.

The operation is due to take place at the Adam Malik Hospital in Medan, North Sumatra. In 2005, doctors at the hospital successfully separated conjoined girls. However, Lubis said the upcoming surgery would be more "complex."

The doctor said the twins both weigh 4.5 kilograms (10 pounds) while they should weigh 10 kilograms for the operation to stand a chance of success. He also said the babies had heart problems and that medical staff would have to monitor their condition for several months.

Surgeons say every case of conjoined twins is different, and every operation is a unique experience in surgical procedures and recovery therapy. Such cases are fraught with dangers such as organ failure and other complications. Separation surgery is considered successful if at least one of the patients survives.

Conjoined twins usually occur in women aged 25 to 40 years, when a single egg, from which identical twins usually develop, fails to divide properly. This occurs in one out of 400,000 births, and the survival rate is 5-25%.

In late December 2008, a conjoined twin named Faith died in London three weeks after her sister Hope died as the result of an operation to separate them.