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President-elect Donald Trump tweeted a stern response to the Berlin Christmas market attack suspect, who was killed by Italian authorities.

Trump quoted a video posted to social media by Anis Amiri, apparently prior to the attack in the German capital, in which the man pledged allegiance to ISIS and promised to "slaughter" "crusaders" with "God's will".

He called the video a "purely religious threat" and urged the United States and its allies to fight back against the terrorist threat.

Amiri was killed in a shootout in Milan, Italy after police sighted him "walking suspiciously" in the early morning hours on Friday.

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