WASHINGTON (CBS DC) — Former CIA deputy director Mike Morell tells “CBS This Morning” that the U.S. needs to respond to the recent ISIS killings of Americans journalists, saying that the U.S. needs a plan to use airstrikes and troops on the ground in Syria to defeat the Islamic State of Iraq and Syria militants.

Morell, CBS News’ senior security contributor, said the U.S. should pursue a “three-pronged strategy” that would include a military strike against ISIS in Iraq, using Iraqi and Kurd forces and backed by U.S. air strikes. Secondly, Morell said that U.S. “needs to go after the leadership of ISIS,” adding that both of those tactics should be underpinned by a political plan in Iraq.

Third, he said the U.S. needs to take on ISIS in the territory they control in Syria, along the border with Iraq. However, he said that fighting ISIS would require a “military on the ground in Syria” in addition to airstrikes, but the only such forces are those of Syrian President Bashar al-Assad – who “we’re fighting as well.”

This comes after the U.S. announcement that it will be adding 350 more troops to help protect the American Embassy in Baghdad and its support facilities in the capital, raising the number of U.S. forces in the country to over 1,000.

President Barack Obama approved the additional troops for protection of American personnel following a request by the State Department and a review and recommendation by the Defense Department, the White House said in a statement.

A recent Pew Research Center poll finds that 71 percent of Americans say Islamic extremist groups like al Qaeda are the top security threats and 67 percent say that ISIS is the largest threat to security. In addition, 54 percent of Americans say that Obama is “not tough enough” on foreign policy and national security issues.

Nearly two-thirds of Americans (66 percent) say they support airstrikes against ISIS in Iraq, according to a recent YouGov poll of Americans public opinion.

“I think the first thing we need to do is to try to pressure the Russians and the Iranians to pressure Assad to step down, that is the best course of action here,” said Morell. “To pressure him to step down to get a new leadership that will work with us against ISIS.”

Morell responded to Obama’s comments that the U.S. will “degrade and destroy” ISIL, saying that the U.S. also needs to push strikes into Syria.

“I think it means strikes in Syria, I think it means a strategy in Syria and I think it means going after the leadership,” said Morell, adding that “guys on the ground” and unmanned aerial vehicles need to target the leadership found through U.S. intelligence within Syria.

Human Rights Watch says told The Associated Press that that the Islamic State fighters killed about 560 men captured at Camp Speicher near the city of Tikrit in June. The “horrific” acts have appeared in videos showing ISIS soldiers massacring scores of Iraqi soldiers in an attempt to sap the morale of dispersing Iraqi security forces.

Morell added that the situation in Libya has worsened by a rise in al Qaeda-like groups.