A top White House national security adviser said Sunday that the United States has strong support from Syrian residents in its effort to destroy Islamic State positions in their country, downplaying protests and arguing that Syrian President Bashar Assad is a common enemy.

"We've seen strong expressions of support from the Syrian opposition for the effort that we're making,” Deputy National Security Adviser Tony Blinken told “Fox News Sunday.”

"The moderate opposition is the common denominator to being able to be a counterweight on the ground to (Islamic State) and then over time also being a counterweight to Assad," he continued. "If you're going to change the dynamics in Syria, if you're going to get to a political transition that moves Assad out, you have to have a strong moderate opposition. We are doing both."

Blinken said the U.S.-led airstrikes on Islamic State targets in Iraq and Syria are going to be “sustained and more effective.”

He also said President Obama has the authority for the use of military force to thwart Islamic State, also known and ISIL and ISIS, based on 2001 and 2002 congressional resolutions, passed after the 9/11 terror attacks.

However, Blinken said Congress “should work to give us a targeted authorization. We welcome that.”

Blinken called it a "significant" development if reports prove true that Muhsin al-Fadhli, the leader of the terrorist organization Khorasan Group, was killed in a recent airstrike. It is possible, Blinken acknowledged, that al-Fadhli is “faking his death.”

Blinken also rejected comparisons between the airstrikes and the Bush-era wars in Afghanistan and Iraq.

"What we’re doing is totally different from the last decade,” he said. “We’re not sending in tens of thousands of American troops on the ground."

On CNN’s “State of the Union,” Blinken said the U.S. and allied Arab States will continue to help train moderate rebel fighters in Syria to remove Islamic State, amid questions about why the U.S. would try to take out the biggest opposition to the Assad regime, which the U.S. also opposes.

“The answer to both Assad and to ISIL actually is the moderate opposition," he said. “They need to be built up so that they can be a counterweight to Assad. And in the near term, they need to be built up so they can work on the ground to help deal with ISIL.”