DOHA - Qatar is working to help free four Americans held hostage in Syria by various armed groups, a Gulf source familiar with the matter said on Monday, a day after the Gulf Arab state's diplomacy helped free a journalist held since 2012.

The source declined to name the four or provide details, and Reuters could not independently verify the assertion, but his account was broadly supported by other sources.

The reported initiative by Qatar coincides with an effort by the tiny state to rebut accusations by some of its Arab neighbors and Western politicians that it supports the most anti-Western militant armed groups in Iraq and Syria. Those allegations followed months of criticism by human rights groups over its treatment of Asian guest workers and charges - denied by Doha - of corruption in its successful bid to host the 2022 World Cup.

The wealthy country, which does back some rebel factions fighting to oust Syrian President Bashar Assad, has mediated the release of foreign and Syrian captives on several occasions in the course of Syria's three-year-old civil war.

Its latest foray into hostage diplomacy brought Sunday's release of Peter Theo Curtis, an American held for nearly two years by al-Nusra Front, an affiliate of al-Qaida.

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