WASHINGTON -- Sen. John McCain says the lack of U.S. aid to Syrian rebels is "shameful," and that helping the opposition would be the biggest blow to Iran in 25 years.

The United States has refused to arm Syrian rebels in part to avoid a proxy fight with Iran and Russia, which back the Syrian government. The crisis in Syria is likely to come up when President Barack Obama meets with Russian President Vladimir Putin in Mexico on Monday.

McCain tells NBC's "Meet the Press" Sunday that members of the Syrian opposition are being "killed and massacred and tortured and raped" and "the fact that Americans aren't helping them is shameful."

Syrian activists say the rebel-held areas in Homs have been under attack for a week.