U.S. Secretary of State John Kerry said he saw an opportunity for progress this week in ending Syria's four-year civil war before meeting Iran's Foreign Minister Mohammad Javad Zarif on the sidelines of the U.N. General Assembly on Saturday.

Western officials said Kerry wants to launch a new initiative to find a political solution to the Syrian conflict, which has taken on a new urgency in light of Russia's military build-up in support of Syrian President Bashar al-Assad and a refugee crisis that has spilled over into Europe.

The new U.S. approach, which officials stressed was in its infancy, could bring Russia, a major ally of Assad, together with countries such as Saudi Arabia, Turkey and Qatar that support Syrian opposition groups against Assad.

"I view this week as a major opportunity for any number of countries to play an important role in trying to resolve some of the very difficult issues (of) the Middle East," Kerry told reporters as he and Zarif posed for photographs.

"We need to achieve peace and a way forward in Syria, in Yemen ... in the region," he said. "I think there are opportunities this week, through these discussions, to make some progress."

Zarif said his priority for the meeting was to discuss the implementation of the July 14 agreement under which Iran agreed to take steps to curtail its nuclear programme in exchange for relief from economic sanctions that have crippled its economy.

"We hope that by its full implementation, its good faith implementation, we can (end) some of the mistrust that has existed over the past many decades," Zarif told reporters.