May 26 (Reuters) - Moody's Investors Service downgraded Qatar's credit rating by one notch on Friday, citing increasing external debt and uncertainty over the sustainability of the country's growth model over the next few years.

Moody's downgraded Qatar's long-term issuer and sovereign debt ratings to Aa3 from Aa2, but changed its outlook on the Middle Eastern nation to stable from negative, citing optimism around the implementation of fiscal and economic reforms.

Separately on Friday, Moody's backed its Aa2 ratings on Kuwait and the United Arab Emirates. The ratings agency raised its outlook on both countries to stable from negative.

(Reporting by Pushkala A in Bengaluru; Editing by Sai Sachin Ravikumar) ((Pushkala.A@thomsonreuters.com; within U.S. +1 646 223 8780, outside U.S. +91 80 6749 6633 ; Reuters Messaging: Pushkala.A.thomsonreuters.com@reuters.net))