Suta Goswami said: Once upon a time in Plastinapura, Pradyota the son of Kshemaka was leading an assembly and meanwhile the great sage Narada arrived there. King Pradyota happily honored him. Plaving him seated on the seat the sage told king Pradyota, "Your father was killed by the mlecchas, therefore he attained Yamaloka or the hellish planet. If you perform a = mleccha-yajna‘, then by the effect of this sacrifice your father will attain the heavenly planets."
Flearing this king Pradyota immediately called the best of the learned Brahmanasand started = mleccha-yajna‘ in Kuruksetra. They built a yajna-kunda which was 16 yojanas in square (128 miles). They meditated on the demigods and offered oblations of mlecchas. There are haras, hunas, barvaras, gurundas, sakas, khasas, yavanas, pallavas,romajas and those who are situated in different dvipas and in kamaru, china and the middle ofthe ocean; all of them were called with the mantra and burnt to ashes. Then he (the king) gave dakshina (donation) to the brahmanas and performed abhiseka. As a result his father Kshemaka went to the heavenly planets. After that he became famous everywhere as a mleccha-hanta or destroyer of mlecchas.
Is this considered authentic?
Does any other scripture talk about sacrificing "Mlecchas"?
Whatever maybe said of "Mlecchas" - scripture has never denied that they are human. So, if genuine, this is a clear statement that human sacrifice was part of Hinduism.