Saying "the time has come for a new policy" in Syria, veteran Republican Sen. John McCain called Monday for U.S.-led airstrikes on President Bashar al-Assad's forces in an effort to protect civilians from a bloody yearlong crackdown by the regime.

"The United States should lead an international effort to protect key population centers in Syria, especially in the north, through airstrikes on Assad's forces," McCain, the top Republican on the Senate Armed Services Committee, said in a speech on the Senate floor.

"To be clear: This will require the United States to suppress enemy air defense in at least part of the country," said McCain, who has repeatedly called in recent weeks and months for a stepped-up U.S. effort to protect Syrian civilians.

Assad has waged a deadly crackdown on opposition to his regime, drawing fierce criticism from Washington and other world powers, though Moscow and Beijing blocked a UN Security Council resolution aimed at halting the bloodshed.

The White House has sharply criticized Russia and China, but has resisted calls to arm the Syrian opposition as premature.

McCain said "the ultimate goal of airstrikes" would be "to establish and defend safe havens in Syria" where Assad's outgunned opponents "can organize and plan their political and military activities" and points for the delivery of humanitarian and military aid "including weapons and ammunition, body armor and other personal protective equipment, tactical intelligence, secure communications equipment, food and water, and medical supplies.

"After a year of bloodshed, the crisis in Syria has reached a decisive moment," McCain said. "Increasingly, the question for U.S. policy is not whether foreign forces will intervene militarily in Syria. We can be confident that Syria's neighbors will do so eventually, if they have not already."

"Some kind of intervention will happen, with us or without us. So the real question for U.S. policy is whether we will participate in this next phase of the conflict in Syria, and thereby increase our ability to shape an outcome that is beneficial to the Syrian people, and to us. I believe we must," the Arizona Republican said.

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"However, it is not clear that the present policy can succeed. If Assad manages to cling to power—or even if he manages to sustain his slaughter for months to come, with all of the human and geopolitical costs that entails—it would be a strategic and moral defeat for the United States. We cannot, we must not, allow this to happen," McCain said.

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