ABC News' Elizabeth McLaughlin reports:

Retired Gen. Carter Ham, who served as commander of U.S. and coalition forces in Mosul, Iraq, from 2004-2005, told ABC News' Martha Raddatz on "This Week" Sunday that initial U.S. airstrikes against ISIS forces advancing toward the city of Erbil have "given pause" to the terrorist group, but that much more will need to be done to halt their progress.

"It will be very difficult without U.S. ground forces or ground forces of others," Ham said when asked if airstrikes will accomplish the President Obama's goals to protect Americans in the region, stop the humanitarian crisis, eliminate ISIS safe havens, and protect Iraqi infrastructure. The airstrikes come two and a half years after the last American troops left Iraq in Dec. 2011.

While President Obama has insisted that ground forces will not be deployed, he did acknowledge that the threat of ISIS will not be solved in a matter of days or weeks.

"This is going to be a long-term project," he said Saturday on the White House lawn before leaving for his Martha's Vineyard vacation.

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Some critics of the president's decision have alleged that the airstrikes will lead to "mission creep" and a return to heavy U.S. involvement in the region. Gen. Ham disagreed, but added that the level of American involvement is still unknown.

"[It] remains to be seen how much support the United States is ready to provide, in my view, first to the Kurdish regional government in Iraq and their armed forces, the Peshmerga, but longer-term to help hopefully a new Iraq government rebuild the Iraqi military," he said.

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Gen. Ham, who was the U.S. commander in Africa during the 2012 attack on the American embassy in Libya, called the U.S. preparation in Iraq "much more significant" than was possible to have in Benghazi at the time.

"Of course there was, at least as far as I am aware, no indications of imminent attack against the U.S. diplomatic facility in Benghazi," he said. "Current circumstance is very different in Iraq where there is an imminent threat. It's very present, and it's known."

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