By CNN National Security Producer Jennifer Rizzo
The violence in Syria will get worse despite increased international pressure, according to the U.S. military's top commander in the Middle East.
Desertions are on the rise in the Syrian military, but President Bashar al-Assad's forces remain viable, Gen. James Mattis, head of the U.S. Central Command, said at a Senate Armed Services Committee hearing Tuesday
"He will continue to employ heavier and heavier weapons on his people," Mattis said. "I think it will get worse before it gets better."
Mattis said al-Assad will be in power "for some time" and is "clearly achieving what he wants to achieve." Though later he said he has no doubt al-Assad will eventually fall, saying the question is "not if, but when."
The general also warned of Syria's weapons stash, saying the country's regime has a substantial amount of chemical and biological weapons, a significant integrated air defense system, and thousands of shoulder-launched anti-aircraft missiles.
Syria's chemical weapons stockpile is one of the largest in the world and if left unsecured there is concern it could fall into the hands of terrorist groups such as Lebanon's Hezbollah organization, Mattis said.
"At the same time they're not easily handled," Mattis cautioned. "Obviously it takes very trained troops to do that...They may end up frying themselves."
The United States has not seen any effort by Assad to use the weapons on his own people.
"But we are watching very closely," Mattis said.
The general speculated that Assad would restrain from unleashing chemical weapons on the opposition because of the international condemnation and probable call to arms it would bring in the international community.
The U.S. administration has not stepped into the conflict or assisted in aiding the Syrian opposition with weapons, citing concerns about the potential for al Qaeda to infiltrate the rebels.
Mattis said the "spectacular IED attacks" launched by the opposition forces provide evidence that the terrorist group is playing a significant role.
Sen. John McCain, R-Arizona, however, still pressed the general and Adm. William McRaven, the U.S. Special Operations commander who was also testifying, on the reasoning behind concerns over arming the opposition.
"Every time I've seen one of these crises the first answer is, 'We don't know who these people are and it could be al Qaeda,' McCain said. "I've heard that (on) Egypt. I've heard that (on) Tunisia. I heard it (on) Libya.
"And you know what that flies in the face of general? People who yearn for liberty and not being under the rule of an oppressive brutal dictatorship. So all of a sudden now we will again assume it's al Qaeda."
McCain urged Mattis and McRaven to use their capabilities to find out who the opposition is.
"A whole lot of people are going to die if we allow the status quo to prevail and the slaughter to continue because, quote, 'we don't know who they are,'" McCain said.
Russia and Iran have stepped into the conflict, aiding al-Assad's forces in ways that have to be taken into account, too.
Russia has provided air defense capabilities, such as missiles and radars, that would make a no-fly zone challenging, Mattis said. And Iran is sending in weapons and eavesdropping capabilities that would help al-Assad's forces find the opposition networks.
"It is a full-throated effort by Iran to keep Assad there and oppressing his own people," Mattis said.
Iran has also flown experts into Damascus to counsel al-Assad, according to the general.
"And they are providing experts who I can only say are experts in oppressing," Mattis said. "They're pretty well schooled. They know how to oppress their own people in Tehran. They've flown them into Damascus to help Assad do the same thing."
Iran has a vested interest in seeing al-Assad stay in power. The two countries are allies in the region, both supporting terrorist groups such as Hezbollah and Hamas.
"It would be the biggest strategic setback for Iran in 20 years, when Assad falls," Mattis said. "Not if, but when. He's going to go."