SANTIAGO -- Chilean sisters conjoined at the thorax born Wednesday are "progressing well," doctors treating them at a Santiago hospital said Thursday.

"The girls are breathing without oxygen today and both have good cardiac signs," AFP quoted Jose Roizen, one of their doctors, as saying.

The girls share heart ventricles: one has the left one and the other the right.

Doctors said they will need to do a series of tests before planning a separation attempt.

The girls were born to an 18-year old mother who spent more than two weeks at Salvador Hospital before giving birth, said a hospital source, who said the mother did not want to be named.

Three similar births of conjoined twins have taken place in Chile over the past six years; none of them survived.

In 1993 doctors successfully separated Jose and Patricio Fuentes who were conjoined, like the girls, at the thorax and shared a heart and a liver. A year after the surgery Patricio fell victim to an infection which left him with mild brain damage.

Jose is in good health and now attends school, according to local press reports.

In Los Angeles Guatemalan one-year old girls conjoined at the head were separated in a delicate and risky operation on Tuesday.