The head of the U.S. Central Command says Iran is the biggest threat to security in the Middle East and that Tehran is working “earnestly” to keep Syrian President Bashar al-Assad in power.

General James Mattis told a Senate hearing Tuesday that Iran has not been dissuaded by international sanctions regarding its controversial nuclear program. He also said Iran has been sending weapons and experts to Damascus to assist Mr. Assad's government.

“Iran and its surrogates continue to orchestrate violence worldwide as evidenced by its plot to kill the Saudi ambassador here in Washington, D.C. Iran presents the most significant regional threat to stability and security. It's reckless behavior and bellicose rhetoric have created a high potential for miscalculation.”

General Mattis highlighted Iran's nuclear program, ballistic missiles, and its use of surrogates to fight what he called a “shadow war.”

At the hearing Senator John McCain repeated his call for military assistance to Syrian rebels and an international effort to provide safe havens for the Syrian opposition.

McCain bristled at those who want to further identify the makeup of the opposition before providing aid, particularly the implication that al-Qaida could be involved. He guaranteed the terror group is not part of the opposition to Mr. Assad.

“Because this conflict is going to go on and a whole lot of people are going to die if we allow the status quo to prevail and the slaughter to continue because 'We don't know who they are.'”

Mattis told McCain the military has seen evidence of al-Qaida having a role in the Syrian opposition. He also said creating a safe haven would take a “significant commitment of resources” because the terrain in Syria does not provide natural protective barriers, such as a mountain range.

Mattis also said Mr. Assad is likely to remain in power under current conditions.

“I think he's going to be there for some time, because I think he will continue to employ heavier and heavier weapons on his people. I think it will get worse before it gets better.”

He said Syria's chemical weapons stockpile – one of the biggest in the world – poses a “potentially serious threat,” but added that he did not think Mr. Assad would use chemical weapons on his own people.