"ISIS is not defeated," Mattis told NBC. "We have got to keep the pressure on ISIS so they don't recover."
Kurdish-led SDF, which has long been considered one of America's most reliable partners in Syria, has played a key strategic role in the campaign against ISIS in the region.
In December, Mattis was "livid" after reading reports about US local allies in Syria being targeted by Turkey following the withdrawal of US troops from Syria, two defense officials told CNN at the time. The officials said what set him off was a report that the Turkish Minister of Defense threatened to kill the US-backed Kurdish allies and put them "in ditches" once the US pulled out.
Mattis told NBC in the clip released Saturday that the current situation is one of "disarray." He said, "Obviously, the Kurds are adapting to the Turkish attacks. And we'll have to see if they're able to maintain the fight against ISIS. It's going to have an impact. The question is, how much?"
Mattis hypothesized that even if the US declares war over and troops are pulled out, ISIS could rebuild. "We may want a war over, we may even declare it over. You can pull your troops out, as President Obama learned the hard way, out of Iraq, but the enemy gets a vote, we say in the military," he said.
The commander of the Kurdish-led Syrian Democratic Forces told a senior US diplomat, "You are leaving us to be slaughtered," and demanded to know whether theUS was going to do anything to protect Syrian Kurdsas Turkey continues its military operation targeting America's Kurdish allies in Syria.
Secretary of Defense Mark Esper said Friday the US is not abandoning its Kurdish allies, but made it clear the US military would not intervene in the fight.
"We remain in close coordination with the Syrian Democratic Forces who helped us destroy the physical caliphate of ISIS, but I will not place American service members in the middle of a longstanding conflict between the Turks and the Kurds, this is not why we are in Syria," Esper told reporters at the Pentagon.
CNN's Ryan Browne and Barbara Starr contributed to this report.