ISIS has reportedly murdered 15 of the Assyrian Christians it captured earlier this week.
Aid to the Church in Need reported that the Islamist group murdered a number of Assyrians it took from Hassake governorate in Syria.
Abbot Emanuel Youkhana passed on the news to various Catholic agencies working in the area, and said the number still being held totalled 350.
Abbot Youkhana said: "Around 15 young Assyrians are martyred. Many of them were fighting to defend and protect the villages and families."
The captured came from a number of villages in the area: 14 people from Tel Hormizd, 81 from Tel Jazira, 21 from Tel Gouran, five from Tel Feytha and three from Qabir Shamiya.
Abbot Youkhana said: "It is believed there are casualties and many Assyrians have been killed in the village."
He added that there was no news about where the families were being held: "Most probably they have been captured and transported to Mount Abdul Aziz, a nearby region controlled by IS."
The charity reports that another source claimed that people in the Sunni Arab village of Bab Alfaraj have been told to attend "a mass killing of infidels" at Mount Abdul Aziz tomorrow.
There are no families left in the 35 Assyrian Christian villages of the area. The only people left in the region are Christian militia fighters who are fighting alongside Kurdish troops.