(Credit: Mark Wilson/Getty Images)

The White House today disputed claims made by a spokesman for the family of slain American journalist Steven Sotloff that he was sold as a hostage to ISIS by the so-called moderate Syrian opposition, the same forces President Obama is now seeking to fund and arm.

"As it relates to the specifics of this matter, based on the information that has been provided to me, I don't believe that is accurate," White House press secretary Josh Earnest said in response to a question from ABC's Jonathan Karl.

Barak Barfi, representing the Sotloffs, told CNN on Monday that the family believes ISIS paid up to $50,000 to "moderate rebel" tipsters who flagged when Steven Sotloff crossed the border into Syria.

"Somebody at the border crossing made a phone call to ISIS, and they set up a fake checkpoint with many people," Barfi told CNN. "Steve and his people that he went in with could not escape."

Earnest said the incident remains under investigation by the FBI. "This is something that they're looking into all aspects of this, including how Mr. Sotloff may have come into the hands of ISIL," or Islamic State of Iraq and the Levant, he said.

Earnest suggested the possibility that Syria's moderate opposition may have been complicit in Sotloff's disappearance does not give Obama pause about his decision to expand American support for the group.

Sotloff was kidnapped in Syria in August 2013. He was beheaded last week by a masked ISIS militant in a video posted online.