UN Secretary-General Kofi Annan said "these kinds of brutal acts do not help anybody."
"My sympathies go to his family and loved ones, and I hope the perpetrators would eventually be brought to justice because we cannot tolerate this kind of behavior in today's world," he said at the UN headquarters in New York.
The New York-based Human Rights Watch called Johnson's slaying "a breach of the most fundamental standards of humanity.
"Holding someone hostage, and then brutally murdering him, is a heinous crime that no political cause can justify," said Joe Stork, Washington director of Human Rights Watch Middle East and North Africa Division.
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