Pope Benedict on Monday reaffirmed Catholic teaching that life begins at the moment of conception, saying embryos are "sacred and inviolable" even before they become implanted in a mother's uterus.
The Pope made his comments in an address to the Pontifical Academy for Life, which is hosting an international congress on scientific aspects and bioethical considerations of "The Human Embryo Before Implantation".
Speaking in Italian, the Pope said the Church had always proclaimed the "sacred and inviolable character of every human life, from its conception to its natural end."
He added: "This moral judgment is valid from the start of the life of an embryo, even before it is implanted in the maternal womb."
By making such a defense of life, the Pope appeared to be trying to cut short any debate that the period between conception and implantation could be seen as a time for legitimate experimentation or manipulation on embryos.
He did not make a distinction between embryos created naturally and those generated outside the womb through in-vitro fertilisation.
The Catholic Church holds that in-vitro fertilisation is morally wrong but scientific advances have presented it with a minefield of ethical issues regarding embryos created outside the womb for artificial implantation.