Washington (AFP) - Iran has withdrawn a "significant number" of its Revolutionary Guards troops from the Syrian battlefield, US Secretary of State John Kerry said Thursday.

Iran is an ally of Syria's President Bashar al-Assad and has sent members of the elite force to act as "advisers" to his forces and to organize militia units with volunteers from Iran, Iraq and Afghanistan.

Tehran also arms and supports Lebanon's Hezbollah movement, which has itself dispatched forces to shore up the Syrian regime against local rebel forces.

But Kerry, speaking two days before a tentative ceasefire is due due to take effect in the long-running civil war, told a congressional committee that Tehran's direct involvement had been reduced.

"The IRGC has actually pulled its troops back from Syria. Ayatollah Khamenei pulled a significant number of troops out. Their presence is actually reduced in Syria," Kerry told US lawmakers, referring to Iran's supreme leader, Ayatollah Ali Khamenei.

"That doesn't mean that they're still not engaged and active in the flow of weapons from Syria through Damascus to Lebanon. We're concerned about that and there's an ongoing concern."

Kerry did not give the source of his information in the open hearing, but he invited the lawmakers "to get the intel briefing."

Tehran has never released official figures for the number of its troops present in Syria, but since October last year more than 100 Iranians have been killed there, including some senior Guards commanders, according to an AFP tally.