The mother of an American journalist held captive by militant group Islamic State released a video on Wednesday appealing directly to the group's leader for his release.

"I am sending this message to you, Abu Bakr al-Baghdadi al-Quraishi al-Hussaini, the caliph of the Islamic State. I am Shirley Sotloff. My son Steven is in your hands," Sotloff said in the video, obtained by the New York Times.

"I ask you to use your authority to spare his life and to follow the example set by the Prophet Mohammed, who protected people of The Book (Christians and Jews)," she said. She added that her son is an "honorable man and has always tried to help the weak."

Sotloff, 31, went missing in Syria last year while covering the conflict there. Islamic State released of video last week in which he could be seen kneeling following the filmed decapitation of fellow journalist James Foley.

Islamic State, which has declared a caliphate in territory it has captured in Syria and Iraq, threatened to kill Sotloff if its demands, which include halting American air strikes against the group, are not met.

"As a mother I ask your justice to be merciful and not to punish my son for matters he has no control over," she said. "I want what every mother wants: to live to see her children's children. I plead with you to grant me this."

Another American writer, Peter Theo Curtis, was released on Sunday after two years held captive by another militant group operating in Syria, the Nusra Front. He said on Wednesday he was emotionally overwhelmed by his welcome home.

The terms of Curtis's release, brokered in part by Qatar, have not been disclosed.

White House spokesman Josh Earnest told reporters in a briefing on Wednesday that "we certainly would call on those who are holding (Sotloff) to release him."

He reiterated that the United States does not pay ransom to release hostages, however, saying it puts other innocent Americans at risk.-Reuters