In a telephone call with U.S. Secretary of State John Kerry, Russian Foreign Minister Sergei Lavrov said arming the rebels "would lead to an escalation in the region, since the U.S. accusations that Damascus has used chemical weapons are not rooted in reliable facts," according to a statement released by the Russian Foreign Ministry.
Washington and Moscow have been deeply divided over how to end the bloodshed in Syria, and the issue is expected to top the agenda when Obama and Putin meet for one-on-on talks Monday at the start of G8 summit in the UK.
"This is a fluid situation. So it is necessary for us to consult with leaders of the G8 about the types of support that we are providing for the opposition," Rhodes said.
Asked about Russia's questioning of U.S. evidence that Syria forces used chemical weapons multiple times, killing between 100 to 150 people, Rhodes said the information provided to Putin's government included samples of sarin gas and other "convincing" evidence.
That evidence, according to Rhodes, includes intelligence reports, eyewitness accounts and "physiological samples" of the nerve agent sarin.
No single piece of intelligence led to the conclusion that the Syrian regime had used chemical weapons, said a senior administration official, who spoke on condition of anonymity. The official was not authorized to speak publicly on the matter.
The finding, according to the official, was a result of looking at a number of instances of suspected use, seeing similar evidence and patterns of usage and coming to the conclusion chemical weapons had been used.
Rhodes acknowledged the differences that remain between the United States and Russia on the Syrian crisis.
The administration believes al-Assad's forces have used saran gas at least eight times in the more than two year conflict, said a U.S. Senate sourced briefed on the matter.
A boost in support by the United States for the rebels could put at risk the gains made by Syrian forces in recent days, especially in central and northern Syria, with the help of Hezbollah fighters from Iran.
In Damascus, an al-Assad loyalist who spoke to CNN's Frederik Pleitgen said he believes the United States is "inventing stories" about the government's use of chemical weapons "because our army is winning."
McCain: Rebels losing fight
The White House announcement comes at a critical time for the Syrian opposition, which has suffered a series of significant losses in recent weeks. Those losses have coincided in large part with the arrival of thousands of Hezbollah Shiite fighters, backed by Lebanon and Iran, to reinforce al-Assad's forces battling the mainly Sunni uprising.
After months of gaining ground, the rebels this month lost Qusayr -- one of their strongholds near the Lebanese border -- which was considered essential for the rebels' supply route.
Until now, the United States has limited its aid to rebels, providing communications equipment, medical supplies and food. Obama signed off on a new package of non-lethal aid in April. That assistance was expected to include body armor, night-vision goggles and other military equipment.
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.