WASHINGTON — Sen. John McCain today became the first senator to call for airstrikes against Syria, saying President Barack Obama has taken too soft a stand against President Bashar Assad and his brutal crackdown on his own people.

The Arizona Republican said the Syrian government’s slaughter of unarmed civilians has likely resulted in war crimes and that its neighbors in the region will intervene militarily, with or without the U.S. From the Senate floor on today, McCain said the United States has a moral and strategic obligation to force out Assad and his loyalists.

“The only realistic way to do so is with foreign airpower,” McCain concluded. “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.”

It was a marked change from McCain’s remarks last month, when he told “CBS This Morning” that the U.S. should find ways to help the Syrian people without putting American “boots on the ground.” Then, he said the options included medical care and technical assistance to safe havens for refugees of the violence.

But in his remarks today, McCain declared it was time to step up militarily and that the U.S. should lead the effort with direct military action. The senior Republican on the Senate Armed Services Committee, McCain was one of the first to call for arming the rebels. That idea has divided members of Congress and drawn concern from the administration about the further militarization of Syria.

The latest McCain proposal is also expected to divide war-weary lawmakers who also opposed the operation in Libya last year.

“The president must state unequivocally that under no circumstances will Assad be allowed to finish what he started,” McCain said on the Senate floor. The Obama administration, he added, should make it clear that “the United States is prepared to use the full weight of our air power to make it so.”