Hilchos Shemittah v’Yoveil 4:27

Under Biblical law, shemittah is not observed in Syria (i.e., lands annexed by Israel) but the Sages prohibited working its land during shemittah as in Israel. This was done to discourage Jews from leaving Israel proper and settling down in annexed lands. Conversely, the Sages required that tithes be taken in Ammon, Moab, Egypt and Babylonia, but shemittah is not observed in these places.

Hilchos Shemittah v’Yoveil 4:28

Shemittah is also observed in Transjordan by Rabbinic enactment. The aftergrowth of Syria and Transjordan may be eaten. That’s because these lands need not be treated more stringently than the parts of Israel that were acquired by the returnees from Egypt.