(Adds Obama, U.N., statement from family, CPJ)
DUBAI/WASHINGTON, Aug 24 (Reuters) - Al Qaeda-linked militants in Syria on Sunday freed an American writer missing since 2012 following what officials said were efforts by the Gulf Arab state of Qatar to win his release.
U.S. Secretary of State John Kerry said in a statement Peter Theo Curtis had been held by Nusra Front, al Qaeda's official wing in Syria whose rivalry with Islamic State has fuelled war among the insurgents themselves.
President Barack Obama, who was briefed on Curtis' release, "shares in the joy and relief that we all feel now that Theo is out of Syria and safe," the White House said.
"But we continue to hold in our thoughts and prayers the Americans who remain in captivity in Syria - and we will continue to use all of the tools at our disposal to see that the remaining American hostages are freed," the statement added.
News of Curtis' release emerged just days after the militant group Islamic State posted a video on the Internet showing one of its fighters beheading American journalist James Foley, who was kidnapped in Syria in 2012.
The United Nations said in a statement "it can confirm that it facilitated the handover of Peter Theo Curtis. He was handed over to UN peacekeepers in Al Rafid village, Quneitra, the Golan Heights, at 6:40 p.m. (local time) on 24 August 2014. After receiving a medical check-up, Mr. Curtis was handed over to representatives of his government."
A senior U.N. official said the negotiations for Curtis' release were handled by the Qataris.
White House national security adviser Susan Rice said in a statement that Curtis was "safe outside of Syria, and we expect he will be reunited with his family shortly."
Curtis, 45, also goes by the name Theo Padnos, according to his family, who described him as a journalist and a writer.
The family said in a statement it was "deeply grateful to the governments of the United States and Qatar and to the many individuals, private and public, who helped negotiate the release of our son, brother and cousin."
"My heart is full at the extraordinary, dedicated, incredible people, too many to name individually, who have become my friends and have tirelessly helped us over these many months," said Curtis' mother, Nancy Curtis, of Cambridge, Massachusetts. "Please know that we will be eternally grateful."
Curtis has written two books, both under the names Theo Padnos: "Undercover Muslim: A Journey into Yemen," a memoir of studying at a madrasa, and "My Life Had Stood a Loaded Gun," a memoir about teaching literature to teenage offenders in Vermont.
The video of Foley's beheading posted on Tuesday, together with a threat to kill another American journalist being held hostage, Steven Sotloff, inspired widespread revulsion in the West and a desire to hunt down Foley's killer.
The U.S.-based Committee to Protect Journalists welcomed the release of Curtis "after nearly two years of harrowing captivity in Syria." It estimates that about 20 journalists are missing in Syria. Many of them are believed to be held by Islamic State.
In a video statement released by Curtis' kidnappers at some point during his captivity, Curtis said he was a journalist from Boston, Al Jazeera reported.
Commenting on his treatment in that video, Curtis said he "had everything" he needed and "everything has been perfect, food, clothing, even friends now." (Reporting by Amena Bakr and Yara Bayoumy in Dubai and Eric Beech in Washington; Additional reporting by Fiona Ortiz in Chicago, Louis Charbonneau at the United Nations and Steve Holland in Edgartown, Mass.; Writing by Eric Beech and Peter Cooney; Editing by William Maclean, Angus MacSwan,Leslie Adler and Andrea Ricci)