Defense Secretary Jim Mattis and White House National Security Adviser John Bolton appeared to set different conditions Monday for the eventual withdrawal of U.S. troops from Syria, although Mattis insisted there was "no daylight" between them.

Mattis has consistently stated that the 2,000 troops in Syria must remain until the last remnants of the Islamic State of Iraq and Syria are eliminated and security is assured in liberated areas, but Bolton has now suggested that U.S. troop withdrawals would be dependent on the withdrawal of Iranian troops from the region.

Bolton said U.S. troops would remain in Syria after the defeat of ISIS to counter Iran, which has sent small numbers of its troops to Syria and backed the Hezbollah militia fighting alongside the forces of Syrian President Bashar al-Assad.

"We're not going to leave [Syria] as long as Iranian troops are outside Iranian borders, and that includes Iranian proxies and militias," he told reporters in New York, where he joined President Donald Trump for United Nations General Assembly sessions.

At the Pentagon, Mattis said the U.S. commitment to Syria is not "open-ended" but gave no estimate on a timetable for withdrawal.

"We are in Syria right now to defeat ISIS and destroy the geographic caliphate and make sure it doesn't come back the moment we turn our backs," he said. "So there is going to be a little while that we've got to work with the locals," once ISIS is defeated.

Currently, U.S. troops and airpower are backing the mostly Kurdish Syrian Democratic Forces in a sweep near the Syrian-Iraqi border focused on the town of Hajin, according to Combined Joint Task Force-Operation Inherent Resolve.

Mattis said the U.S. is also intent on resuming peace talks in Geneva led by United Nations special envoy Staffan de Mistura to end the five-year-old civil war and negotiate Assad's removal from power.

At the Pentagon, Mattis he and Bolton were aligned on Syria, adding, "I think we're on the same sheet of music," but he deferred questions on their possible policy differences to the White House.

The seeming rift with Bolton follows reports -- vehemently denied by Mattis -- that he is considering resigning after the November elections because of differences with the White House on the national security agenda.

On another Syria issue, Mattis and Bolton clearly are in agreement. They are against Russian sales of S-300 anti-aircraft missiles to Syria.

"We think introducing the S-300s to the Syrian government would be a significant escalation by the Russians," Bolton said. The sale, he said, is "something that we hope, if these press reports are accurate, they would reconsider."

On Monday, Russian President Vladimir Putting phoned Israeli Prime Minister Benjamin Netanyahu to tell him that Syrian air defenses are being upgraded to "prevent any potential threat to the lives of Russian servicemen, who are on a mission to fight international terrorism."

Last week, a Russian reconnaissance IL-20 aircraft was shot down off the Syrian coast near the port of Latakia by an errant Syrian missile fired at attacking Israeli F-16s, killing all 15 aboard the Russian aircraft.

Bolton said that a political settlement is the only way out of Syria's civil war, but the Russian sale of the S-300s will make the process more difficult.

At the Pentagon, Mattis said the addition of S-300s will strengthen the regime and make Assad less likely to give up power.

-- Richard Sisk can be reached at Richard.Sisk@Military.com.