This comes 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 neighbours 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 al-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 Qaeda.
“Four other Americans who have gone missing in Syria have now been located, and Qatar is working to free them,” the Gulf source told Reuters on condition of anonymity. He said the hostages were being held by “various groups” but declined to give details.
Qatar’s foreign ministry declined to comment.
A Doha-based source close to the Qatari Government said without elaborating that Washington was working with Qatar to try to free a number of U.S. hostages in Syria.
A rebel commander in Syria reached by Skype from Beirut told Reuters that Qatar was continuously trying to secure the release of captives of all nationalities.
Under a policy of international self-promotion, Qatar has for years also played peace broker in disputes from Somalia to Lebanon, and has irritated conservative neighbors by supporting Arab Spring revolts and bankrolling Islamist influence.
Qatar this week argued its aims in the region were peaceful and humanitarian, issuing a statement condemning what it called Foley’s “barbaric” murder.
Qatari Foreign Minister, Khaled al-Attiyah, said earlier this year his country’s mediation had secured the release in March of 13 Greek Orthodox nuns after more than three months of being held by Islamist fighters. The British-based Syrian Observatory for Human Rights identified the rebels who took the nuns as the Nusra Front.
Before that, Qatar helped win the release of 11 Lebanese Shi’ites in October 2013 after 17 months in captivity in Syria.