"Despite the efforts that are made to deny it, science continues to show us that the embryo is a human being in the making," Elio Sgreccia, emeritus head of the Pontifical Academy for life, told Radio Vatican.
Embryos are "sacrificed to extract the stem cells from them," Sgreccia argued, saying that such a technique "from an ethical point of view can only receive a negative judgment."
US biotech firm Geron Corp announced on Friday it had been cleared by the US Food and Drug Administration (FDA) to carry out the world's first clinical trials using stem cells derived from human embryos.
It plans to test the therapy on patients paralysed by spinal-cord injury.Embryonic stem cells are highly versatile, primitive cells capable of developing into any tissue of the body.
The trial's goal is to inject cells into the spines of paralysed volunteers in the hope that this will prompt damaged nerve cells to regrow, enabling the patients eventually to recover feeling and movement.
The Geron trial won initial FDA approval in January last year but was put on hold because of safety concerns.
The Catholic Church is opposed to therapeutic research on stem cells drawn from human embryos, although not if the stem cells are drawn from the umbilical cord or parts of the adult body such as the intestine or the retina.