The United States has no information indicating beheaded American journalist Steven Sotloff was "sold" to Islamic State militants by moderate Syrian opposition rebels, White House spokesman Josh Earnest said on Tuesday.

Sotloff family spokesman Barak Barfi told CNN on Monday night the family believed Islamic State paid up to $50,000 to rebels who told the militant group the 31-year-old journalist had entered Syria.

"Based on the information that has been provided to me, I don't believe that is accurate," Earnest told a news briefing.

He cited an FBI investigation of Sotloff's death, including "how Mr. Sotloff may have come into the hands of ISIL," another acronym for Islamic State.

The militants released a video on Sept. 2 showing the beheading of Sotloff, who was kidnapped in Syria in August 2013.

Barfi told CNN's "Anderson Cooper 360" that Sotloff's family learned from unidentified "sources on the ground" that a member of a moderate Syrian rebel group contacted Islamic State militants about Sotloff. He confirmed the comments to Reuters on Tuesday.

Barfi said the family was disappointed with the Obama administration's handling of the situation, but he did not elaborate and said the family would soon speak for itself.

President Barack Obama is seeking to increase aid to moderate Syrian opposition groups who are fighting to oust President Bashar al-Assad and are also seen as a tool against Islamic State.

The militants have seized territory in Syria and Iraq and declared an Islamic caliphate.

In an Aug. 19 video showing the beheading of another American journalist, James Foley, the group said it was retaliating for U.S. airstrikes on its insurgents in northern Iraq.

The United States resumed airstrikes in Iraq in August for the first time since the pullout of U.S. troops in 2011.

Obama may seek to expand the strikes to Syria and is presenting his plan for fighting Islamic State to Congress on Tuesday and the American public on Wednesday night.