Tillerson sees long-term US presence in Syria

"Continued strategic threats to the US from not just ISIS and al Qaeda persist, and this threat I am referring to principally is Iran," Tillerson said, adding, "In short, Syria remains a source of severe strategic threats."

Tillerson spoke at the Hoover Institution on the campus of Stanford University before an audience that included former Secretaries of State George Shultz and Condoleezza Rice.

He said President Donald Trump's administration was implementing a new strategy on Syria that would achieve "key end states" for the carnage engulfing the country -- which has left a half million people dead -- and would ultimately lead to a political resolution without Syrian President Bashar al-Assad remaining in power.

The continued presence of ISIS in Syria, despite the loss of a majority of its territory there, is necessary, Tillerson said, to avoid a repeat of the security vacuum that followed the US withdrawal from Iraq in 2011 under the Obama administration and the conditions that allowed ISIS and other terrorist groups to flourish in the region.

"ISIS presently has one foot in the grave, and by maintaining an American military presence in Syria until the full and complete defeat of ISIS is achieved, it will soon have two," he said.

"We understand that some Americans are skeptical of continued involvement in Syria," Tillerson said, but he argued such engagement is necessary to assure ISIS "do not present a threat to the homeland, and do not resurface in a new form."

Similarly, Iranian influence inside Syria through Tehran's use of proxy forces alongside its own military presence remains a reality that must be checked, Tillerson said.

"As a destabilized nation, and one bordering Israel, Syria presents an opportunity that Iran is all too eager to exploit," he said. "We must ensure that the resolution of this conflict does not put Iran closer to its goal of controlling the region."

And Assad's continued military operations against his own citizens -- including the use of chemical weapons, which provoked a military response by the United States last April -- have created a humanitarian crisis of millions of refugees and only add to the instability in Syria, the secretary said.

"The persistence of Assad in power will fuel additional conflict and increase the risk of additional chemical weapons use," Tillerson said.

He reiterated US support for the UN-led process that envisions a unified and stable Syrian state without Assad in power, and called on Russia to use its influence with Assad's regime to reach that end.

"The Assad regime clearly looks to Russia as a guarantor of its security," Tillerson said.

He added that Russia "has a meaningful role to play in persuading the Assad regime to engage constructively in the Geneva process."

Over the long term, the United States and its allies must participate in "stabilization initiatives" across Syria such as de-mining and establishing stable electricity, safe drinking water and other elements of a functioning state, which would be State Department and USAID initiatives backed up by the US military presence, Tillerson said.