Steven Sotloff, the second American journalist beheaded by Isis (now known as the Islamic State), was sold to the terror group by a "moderate" rebel group in Syria for tens of thousands of US dollars, his family's spokesman has confirmed.

In an interview with CNN, Barak Barfi said that the joint US-Israeli citizen's location at the Syrian border was sold for "between $25,000 and $50,000" in a tip-off to the Islamic militants, according to information obtained from credible "sources on the ground".

"For the first time, we can say Steven was sold at the border," Barfi said.

Barfi told how Sotloff was kidnapped after false rumours, spread by "activists", that the journalist had been party to the bombing of a Syrian hospital. This rumour led to his kidnap and sale to Islamic State militants.

"Steven's name was on a list that he had been responsible for the bombing of a hospital. This was false, activists spread his name around.

"Somebody at the border crossing made a phone call to Isis and they set up a fake checkpoint with many people and Steven and his people that he went in with could not escape," he said.

Barfi said that the group which sold Sotloff to the radical Islamists were likely part of wider rebel factions which US President Barack Obama considered supporting in the hope of ousting Syrian President Bashar al-Assad from power.

"We believe that these so-called moderate rebels that people want our administration to support, one of them sold him probably for something between $25,000 and $50,000 to Isis and that was the reason he was captured," Barfi said.

Political "pawns"

Barfi continued to detail the poor relationship between Sotloff's family and the Obama administration for using the journalist and James Foley - another journalist beheaded by the group - as "pawns" in a wider political game and ignoring the one request from the Sotloff family after his murder.

"The relationship between the Sotloff family and the administration was very strained... we do not believe that they gave us the cooperation we need.

"Once Steve appeared in that video the Sotloff family made one simple request of the administration and they were rebuffed on that," he claimed. He said that he was not able to reveal what the request was as it could compromise the safety of the hostages still held by the Islamic State.

The family spokesman then accused the US government of leaking information to the press, some false and some in bad taste, such as the details of the torture faced by the captives. He warned that the Sotloff family would go public to "set the record straight" if such leaks continued.

"We know that the intelligence community and the White House are enmeshed in a larger game of bureaucratic infighting and Jim and Steve are pawns in this game and that's not fair and if there continues to be leaks the Sotloff family will have to speak out to set the record straight," he said.

Sotloff, who worked as a journalist for Time and Foreign Policy magazine, had been threatened with execution by IS in revenge for US air strikes on the group's positions in northern Iraq. He had been held hostage by IS for more than a year.

British and American intelligence officials have confirmed that they are certain of the identity of Sotloff's killer, the "Jihadi John" executioner, and are set to unmask him "within days".