The advanced air defense weapons Russia has provided to Syria's regime would make it difficult to establish a no-fly zone there as part of an effort to help the rebellion, the top U.S. commander in the Middle East told senators Tuesday.

Marine Gen. James Mattis, head of U.S. Central Command, declined to detail any military options the Pentagon has developed for action against the regime. But he told the Senate Armed Services Committee that it would take a significant military commitment to create even safe havens in Syria where aid could be delivered, as Sen. John McCain suggested Monday.

Mattis said he has not been directed to do detailed planning on how to establish safe havens in Syria where opposition forces could be trained. But he said that since there is no protective terrain, a large number of international military troops would have to be used to create a security barrier.

President Barack Obama said Tuesday that unilateral military action by the United States against the Syrian regime would be a mistake, and that the situation in Syria is more complicated than it was in Libya.

Senators repeatedly pressed for options to stem the brutal offensive against the Syrian people by President Bashar Assad's regime. And they questioned Iran's involvement there, as well as alleged efforts by Tehran to develop nuclear weapons.

McCain said he is growing angry over the argument that the U.S. and others want to figure out who the Syrian opposition is before providing greater aid to them. A lot of people will die before that happens, he said.

"I suggest we find out who these people are and I guarantee you that you will find out that it's not al-Qaida," said McCain. "It's people who have the same yearnings that are universal and that's freedom, democracy and our God-given rights."

The Obama administration and other international leaders have opposed military intervention in Syria, and instead have pushed instead for increased sanctions.

Mattis acknowledged that Assad is gaining momentum on the battlefield.

"I think he will continue to employ heavier and heavier weapons on his people. I think it will get worse before it gets better," Mattis said.

He added that if there was a decision to intervene in Syria, Iran would use surrogates to foment violence and back the regime, but would likely try to avoid overt military support.

Asked by Sen. Joseph I. Lieberman, I-Conn., what Iran is doing now there, Mattis said it is providing weapons to suppress the opposition, listening capabilities to figure out where the rebel networks are operating, and experts on oppression.

According to Mattis, Syria has a "substantial" chemical and biological weapons capability and thousands of shoulder-launched missiles. Until now, the U.S. military has largely declined to describe the expanse of weapons that President Bashar Assad's regime has at its disposal.

The prospects of a civil war are rising in Syria, he said, but the "options available to address the situation are extremely challenging."

U.S. officials argue that unlike the military campaign in Libya last year that ousted Moammar Gadhafi, a military campaign in Syria would be far more difficult, would not get the backing of the U.N. Security Council and would be hampered by a less coordinated opposition force. Russia and China have blocked efforts by the Security Council to punish Syria.

Senators also questioned whether sanctions against Iran are working. Mattis said that military action against Iran would only delay efforts there to develop nuclear weapons, and only the Iranian people can force real change there.

Mattis' comments come as the Obama administration meets with Israeli leaders this week to discuss the escalating Iranian threat and the possibility of a pre-emptive strike by Israel.

In other issues, Mattis warned against efforts to scale back the Navy's presence in the embattled region, saying threats from Iran and elsewhere will require more ships and maritime missile defense capabilities.

Against a backdrop of roughly $500 billion in Pentagon budget cuts over the next decade, Mattis said the U.S. must use its Navy and special operations forces to maintain a smaller but still strong military presence in the Middle East as the wars in Iran and Afghanistan end.

"The stacked Iranian threats ... of ballistic missiles, long-range rockets, mines, small boats, cruise missiles and submarines demand stronger naval presence and capability to protect vital sea lines of communication," Mattis said.


Lolita C. Baldor can be followed on Twitter at http://twitter.com/lbaldor