video seems to show militants in libya holding one group of at least 16 captive on a beach and 12 others in a desert .
before the killings a masked fighter in black brandishes a pistol as he vows to kill christians if they do not convert .
ethiopia unable to confirm its citizens were killed by militants in the footage but condemned the ` atrocious act '
it comes two months after 21 egyptian christians were beheaded by extremists in a similar video from libya .