Sen. John McCain, who has repeatedly pushed the Obama administration to step up its support for the rebels, told CNN on Friday that they need anti-tank and anti-aircraft weapons.
Asked whether the rebels were losing the fight, the Arizona Republican said: "Absolutely, there's no doubt about it."
He also called for taking out al-Assad's air assets to create a safe zone for the Syrian opposition.
"I know that we have the military capability to impose a 'no-fly' zone, to crater their runways and their fixed installations where fuel and parts are, and establish a 'no-fly' zone with Patriot missiles," McCain said.
"And if we can't do that, then the question ought to be asked to the American taxpayer -- to the Pentagon, 'What in the world are we wasting tens of billions of dollars for defense for if we can't even take care of this situation?'" McCain said.
Rhodes, however, indicated that a "no-fly" zone was unlikely, saying it would be "dramatically more difficult and dangerous and costly" to enforce one in Syria compared to the one NATO forces imposed with U.S. backing during Libya's civil war.
Libyan rebels had control of large portions of the country, unlike the Syrian rebels, he noted, and the Libyan military had fewer air-defense systems. He added that a "no-fly" zone "is not a silver bullet."
U.S. defense officials are not reviewing any new or updated options for a no-fly zone, two Pentagon sources said.
Even if U.S. planes monitored a no-fly zone along the Syrian-Jordanian border, the Syrian regime could attack targets in southern Syria using long range artillery or Scud missiles, a senior Pentagon official said.
Syria has long maintained that rebels, not government forces, are behind the use of chemical weapons. It also went to the United Nations with its claims, but al-Assad would not allow U.N. inspectors into the country to try to verify the claims.
Analysts believe the Syrian government may have one of the largest stockpiles of chemical weapons in the world. Specifically, the supply is believed to include sarin, mustard and VX gases, which are banned under international law. Syria has denied the allegation.
Sarin gas can be hard to detect because it is colorless, odorless and tasteless. It can cause severe injuries -- including blurred vision, convulsions, paralysis and death -- to those exposed to it
In recent months, reports have repeatedly surfaced that Syrian forces have moved some of the chemical weapons inventories, possibly because of deteriorating security in the country, raising fears the stockpile could fall into the hands of al Qaeda-linked groups working with the opposition should al-Assad's government fall.
U.S. officials have been closely monitoring Syria's chemical weapons stockpiles and are certain they remain in the control of al-Assad's regime, Rhodes said, adding that it would be too dangerous to destroy the chemical weapons stockpiles from afar.
As recently as last week, the French foreign minister said sarin gas had been used several times in the Syrian civil war, citing results from test samples in France's possession.
In early May, the head of the U.N. Independent International Commission of Inquiry on Syria said that evidence points to the use of sarin by Syrian rebel forces. But the commission later issued a news release saying it "has not reached conclusive findings as to the use of chemical weapons in Syria by any parties to the conflict."