Not a teacher
But is a conjunction that indicates a contradiction or exception to the previous clause. In this sentence, "but" and "as" are not actually a combined phrase.
You could change the sentence to say:
"Even though the debate over human cloning is just beginning, as science advances, it could become the most ethical dilemma in the 21st Century."
I rewrote the sentence, using "even though" to essentially replace "but." The sentence means that we can already see evidence that human cloning will be a controversial issue even though it is a relatively new issue.
I hope I was helpful. Sorry I don't speak French, otherwise I would attempt to translate.
Student or Learner