Defense Secretary Leon E. Panetta said Sunday that the fighting in Aleppo, Syria's largest city, will be "a nail in Assad's coffin," for Syrian President Bashar al-Assad.

He was traveling to North Africa and the Middle East where he held a series of meetings with key leaders in the region.

"The key right now is to continue to bring that pressure to bear on Syria to provide assistance to the opposition and to provide whatever humanitarian aid we can to assist the refugees," Panetta told reporters on his flight to Tunisia.

The fighting in Aleppo has escalated over the past week. Hundreds of thousands of people have fled the violence in Syria to neighboring countries of Turkey, Jordan and Lebanon creating a humanitarian crisis. U.S. Officials are closely working with neighboring nations to assist with the humanitarian crisis, Panetta said.

He added that chemical and biological stockpiles in Syria must be secured.

"We've been in close coordination with countries in the region to ensure that this is happening," the secretary said.

Sara A. Carter is The Washington Examiner's national security correspondent. She can be reached at