Possibly, Jesus was telling them early on to stay away from the Gentiles, you know, to practice on attaining a base of converted Jews. But 1 Peter 5:13, using the term Babylon (which makes no sense if taken literally since it was in complete ruins) as reference to Rome (as it appears through the later New Testament) seems to indicate differently.
but then the entire body of the people of Israel remained in that country; wherefore there are but two tribes in Asia and Europe subject to the Romans, while the ten tribes are beyond Euphrates till now, and are an immense multitude, and not to be estimated by numbers.
You can see here the entire quote in the works of Josephus Antiquities, Book XI, Chapter V, Paragraph 2.
An immense number to me would indicate more than an area in complete ruins....and deserted. Remember, Babylon was also considered a territory as well as a city. Many Jews did not return (600 B.C.) with Ezra and Nehemiah. This was where the majority of the "Talmud" was developed. See Talmud