Washington, Jan 18 (IANS): The US must remain both diplomatically and militarily engaged in Syria to protect its own national security interests, Secretary of State Rex Tillerson has said.

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

Tillerson was speaking on the campus of Stanford University before an audience that included former Secretaries of State George Shultz and Condoleezza Rice, CNN reported.

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 the IS 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 and the conditions that allowed the IS and other terrorist groups to flourish in the region.

"The IS 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," Tillerson said.

"We understand that some Americans are sceptical of continued involvement in Syria," Tillerson said, but he argued such engagement was necessary to assure the IS "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.

"Assad's continued military operations against his own citizens... have created a humanitarian crisis of millions of refugees and only add to the instability in Syria," the Secretary 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.