An American journalist kidnapped nearly two years ago has been freed in Syria following Qatari mediation and handed over to UN peacekeepers in the Golan Heights.

Peter Theo Curtis was handed over to UN peacekeepers in the village of al-Rafid, Quneitra, on Sunday. He has since been turned over to representatives from the US government after undergoing medical check-up, the UN said.

Curtis' family thanked both the governments of the US and Qatar, as well as others who helped negotiate his release.

According to a statement from his family, Curtis was captured in October 2012 and was reportedly held by the al-Nusra Front or by splinter groups allied with the al-Qaeda-affiliated group.

We are all relieved and grateful knowing that Theo Curtis is coming home after so much time held in the clutches of [al-Nusrah Front] John Kerry, US Secretary of State

US Secretary of State John Kerry said he was relieved Curtis was returing home, before laying blame on al-Nusra Front for the kidnapping.

"We are all relieved and grateful knowing that Theo Curtis is coming home after so much time held in the clutches of [al-Nusrah Front], he said.

Kerry also said the US was using "every diplomatic, intelligence, and military tool" available to release other Americans held hostage.

Qatar's foreign ministry has also issued this statement about its role in the release of Peter Theo Curtis.

"Qatar exerted relentless efforts to release the American journalist, out of Qatar's belief in the principles of humanity and its keenness on the lives of individuals and their right to freedom and dignity."

Footage of the American was released on June 30, showing a disheveled Curtis with long hair and beard, but appearing to be in good health.

Speaking in a video obtained by Al Jazeera, Curtis read from a prepared script stating his name and profession, saying he was a journalist from Boston, Massachusetts.

Commenting on his treatment, Curtis said he "had everything" he needed and "everything has been perfect, food, clothing, even friends now".

In June, a 27-year-old German held hostage by Islamic State fighters was released after Berlin reportedly made a deal with the group.

According to German newspaper die Welt am Sonntag, "something was given in return for his release".

Earlier this year, 13 nuns were freed after being kidnapped by Syrian rebels following Lebanese-Qatari mediation, ending a three-month ordeal in a rare prisoner exchange with the government.

Curtis' release comes just days after the beheading of US journalist James Foley, who was captured in Syria in 2012.

On Tuesday, the Islamic State group released a graphic video on social media sites, showing one of its fighters beheading Foley, which it said was revenge for US raids on its territory.

In the video, titled "A Message To America," the group claimed to be holding another US journalist, and said his life depended on US President Barack Obama's next move.

"The life of this American citizen, Obama, depends on your next decision," said a masked man in the video posted on social media sites, speaking English with a British accent as he held a prisoner the video named as Steven Sotloff.

According to the Committee to Protect Journalists (CPJ), at least 67 journalists have been killed in Syria since the start of the uprising, highlighting the risks of reporting from the country.

Dozens of journalists covering the civil war have been seized since the conflict began in March 2011, with many others still missing.