WASHINGTON, July 7 (Reuters) - The United States, Russia and regional countries have reached a ceasefire deal in southwestern Syria, one of the combat zones in a six-year civil war, a U.S. official said on Friday.

U.S. Secretary of State Rex Tillerson was expected to provide more details of the deal to reporters in Hamburg later on Friday, the official said.

Russia and Iran are the main international backers of Syrian President Bashar al-Assad while Washington supports some of the rebel groups fighting for his ouster. "Still a lot of work to be done," the U.S. official said.

Before leaving for Hamburg, Tillerson said the United States was prepared to discuss joint efforts with Russia to stabilize Syria, including no-fly zones, ceasefire observers and coordinated deliveries of humanitarian assistance.

Tillerson was present at a meeting in Hamburg between U.S. President Donald Trump and Russian President Vladimir Putin, along with Russian Foreign Minister Sergei Lavrov.

(Writing by Yara Bayoumy and Alistair Bell; Editing by Jeffrey Benkoe) ((yara.bayoumy@thomsonreuters.com; tel: +1202 779 1681; Twitter: @yarabayoumy; Reuters Messaging: yara.bayoumy.thomsonreuters.com@reuters.net))