The United States said Monday that it had asked Qatar in advance not to pay a ransom for the release of US hostage Peter Theo Curtis, who was freed by an Islamic rebel group in Syria.

Curtis's family said the Qatari government had repeatedly reassured them that it had not won his freedom through a cash payment, as debate mounted over the US policy of refusing to pay ransoms to extremist groups.

Curtis was freed on Sunday after what the White House said were its efforts to facilitate contacts between the Curtis family and the Qatari government.

"The United States government certainly did not ask the Qataris to pay a ransom. In fact, we asked the Qataris, consistent with our long-standing policy, to not pay a ransom for Mr Curtis," Earnest said.

"That all said, we are grateful in knowing that Mr Curtis is coming home after so much time held in captivity in Syria."

Curtis was freed less than a week after grisly footage emerged of the execution of another US hostage, journalist James Foley by the Islamic State -- a separate group to the one that held Curtis.

Curtis's relatives said they were repeatedly told that the Qatari government was trying to secure the 45-year-old's release with good faith negotiations and not a promised ransom payment.

Foley's killing ignited fresh debate in the United States over the issue of ransoms. Some European states are suspected here of offering cash payments for kidnapped citizens in war zones like Syria in a manner that American officials say only encourages more hostage taking.

Washington sticks to a policy of never paying ransoms, saying doing so would endanger Americans all over the world.

State Department spokeswoman Jen Psaki said that Curtis was handed over to UN peacekeepers in the Golan Heights late Sunday and taken to Tel Aviv by US government officials.

She could not say when he would return to the United States.

According to a family statement, Curtis was captured shortly before he crossed into Syria in October 2012 and was held since then "by the militant group Jabhat al-Nusra or by splinter groups allied with Jabhat al-Nusra."

The Islamic State and Al-Nusra are rooted in Al-Qaeda in Iraq but the two groups have been openly at war with each other in Syria since early this year.