The mother of a US hostage who has been threatened with death by jihadist militants pleaded for his life on Wednesday amid mounting fears for Americans captured in Syria.

Last week's murder of journalist James Foley by the so-called Islamic State has focused attention on other American hostages, including 31-year-old freelancer Steven Sotloff.

Another journalist, Peter Theo Curtis, returned home Tuesday after nearly two years in captivity, but US media reported the Islamic State is still holding a 26-year-old female aid worker.

The masked militant seen beheading Foley in a video released by the group also threatened to kill Sotloff unless US President Barack Obama orders a halt to air strikes against the group.

- Mother's appeal for mercy -

On Wednesday, Sotloff's mother Shirley issued a video appeal in which she directly addressed IS leader Abu Bakr al-Baghdadi, begging him to spare her son from the same grisly fate.

"My son Steven is in your hands," she said, looking tired and tense but controlled. "He is a journalist who made a journey to cover the story of Muslims suffering at the hands of tyrants."

"As a mother, I ask your justice to be merciful and not punish my son for matters he has no control over."

A spokesman for the White House could not say whether the US administration had advised the family for or against making the video appeal.

"She obviously, as is evident from the video, feels desperate about the -- about the safety and well-being of her son, and understandably so," Josh Earnest said.

"And that is why our thoughts and prayers are with Mr. Sotloff's family at this very difficult and trying time.

"As you know, this administration is deeply engaged in doing everything we can to seek the return of every American who is currently being held in that region."

Sotloff disappeared while reporting in Syria in August 2013. His apparent kidnapping was not widely reported until he appeared on the video released last week by the Islamic State.

"We have not seen Steven for over a year and we miss him very much. We want to see him home safe and sound and to hug him," Shirley Sotloff said.

"Since his capture, I have learned a lot about Islam. I've learned that Islam teaches that no individual should be held responsible for the sins of others," she said.

"Steven has no control over the actions of the US government. He's an innocent journalist. I've always learned that you, the Caliph, can grant amnesty. I ask you to please release my child.

"I ask you to use your authority to spare his life and to follow the example set by the Prophet Mohammed."

The Islamic State, a jihadist militant group, has declared the stretch of territory it controls in eastern Syria and northern Iraq to be the heartland of a new caliphate and Baghdadi the leader of all Muslims.

- Other hostages -

The appeal for Sotloff came as ABC News and other US media reported that a 26-year-old female American aid worker is also being held by the Islamic State.

Foley, in a letter to his family committed to memory by a released Danish hostage, said that at one point he was being held with 17 people -- although several Europeans have been released.

Although their governments deny it, it has been widely reported that European countries have paid ransoms to the Islamic State for the safe return of their nationals.

The United States traditionally refuses to negotiate with groups it considers terrorists and it has continued its air strikes against IS targets in Iraq since Foley's murder.

There was better news, however, for another US journalist who was captured in Syria by a different jihadist group, the Al Qaeda-linked Al-Nusra Front, in October 2012.

Curtis, 45, arrived in Boston from Tel Aviv after having been released on Sunday to UN peacekeepers in the Golan Heights. Qatari agents negotiated his freedom from the rebels.

The freelance journalist and author made a brief statement Wednesday to reporters outside his mother's house in Cambridge, Massachusetts.

"I had no idea when I was in prison... that so much effort was being expended on my behalf," he said. "Now having found out, I am just overwhelmed with emotion."

Curtis's family has said the Qatari government had reassured them that it had not won his freedom by paying a ransom.

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