WASHINGTON (AFP) - Several viable human pregnancies are in progress in which cloned test-tube embryos were implanted in host mothers, but the rate of miscarriage is higher than normal, the head of a human cloning firm said.

"Yes, we have viable pregnancies, that is to say three months or more," French chemist Briggitte Boisselier, president of the firm Clonaid, told AFP in a telephone interview from Clonaid headquarters in Las Vegas, Nevada.

She said the implants of blastocysts, or embryos four to five days from conception, had begun last February and March, but she declined to detail how many were done, the stages they were at or the outcomes.

She said miscarriages had occurred, "as they do in test tube conceptions."

Several specialists in the field, including biologist Rudolf Jaenisch of Massachussetts Institute of Technology's Whitehead Institute, say such experiments are not only doomed to failure, but are "irresponsible and repugnant."

Jaenisch has long spoken out against human cloning, a technique that has high failure rate in animals and resulted in "a veritable gallery of horrors" among aborted fetuses and live births.

They include, he said, congenital malformations, physical deformities, immune system deficiencies and premature aging.

Among the small number of cloned animals that live more than a few days, many suffer defects or disease including pneumonia, liver deficiency, obesity and premature aging, said Jaenisch.

However, said Boisselier, examination of aborted cloned fetuses would have shown no abnormalities, implying the problem may have been in the birthing rather than the gestation.

"We're trying to understand," she said.  "Many factors can come into play.  The woman might not have been in condition to receive fetus."

She would not say what the failure rate was.