MIAMI - The family of Steven Sotloff, the second American journalist beheaded by Islamic State militants, said on Wednesday he was "a gentle soul", and challenged the group's leader to a debate on the peaceful teachings of the Muslim holy book, the Koran.

The group, which has captured territory in Syria and Iraq, released a video on Tuesday of Sotloff being beheaded. US officials confirmed its authenticity on Wednesday. President Barack Obama vowed to "degrade and destroy" the group.

Barak Barfi, a friend of Sotloff who is serving as family spokesman, began a prepared statement from the family in English, remembering the slain journalist. Barfi ended the statement with off-the-cuff remarks in Arabic, saying "Steve died a martyr for the sake of God."

He then challenged Islamic State leader Abu Bakr al-Baghdadi to debate Islam, saying, "Woe to you. You said the month of Ramadan is the month of mercy. Where is your mercy?"

"God does not love the aggressor," added Barfi, who is an Arabic scholar and research fellow at the New America Foundation think tank in Washington.

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