Russia's increased military support for Bashar al-Assad, which in recent weeks has expanded to a campaign of airstrikes, has not significantly improved the dictator's strategic outlook in the Syrian civil war, the U.S. intelligence community believes.

Moscow may not be able to prop up Assad's regime much longer given the strength of his opponents, which include rebels fighting his regime as well as the ISIS terror group, which controls large swaths of territory across Iraq and Syria, unidentified intelligence officials told CNN.

"Many of the (anti-Assad) forces that remain in key areas are battle-hardened and experienced fighters," an unnamed source told the news channel. "This includes not only (ISIS), but the moderate opposition, Nusrah and Ahrar al Sham."

Russia's airstrikes, which have earned President Vladimir Putin much criticism as they seemed to target dissidents rather than ISIS terrorists, have not been particularly effective, the source added.

"Airpower alone is unlikely to turn the battlefield in Assad's favor," the official explained. "His army is depleted and demoralized. While some tactical swings along the front lines may favor the regime in the near-term, pro-regime forces face significant challenges in loosening the opposition's grip on territory it has held for months, and in some cases years."

Meanwhile, lawmakers in Congress are increasingly weary about sharing U.S. intelligence on ISIS with Putin, Al-Monitor reported, and some members of the bipartisan House Homeland Committee have voiced particular concern over Iraq's announcement of an intelligence-sharing agreement with Russia, Iran and Syria's pro-Assad forces.

"I will say the one thing we have in common with the Russians is our dislike for the terrorists," Texas Republican Mike McCaul, the panel's chairman, told the Middle East-focused website. "The complication is their obvious support for Assad. And as long as Assad remains in power, he remains a magnet for the jihadists."

Meanwhile, another unidentified senior U.S. officials noted that much would depend on Moscow's true goals in the conflict.

"We're just at the beginning of trying to understand what the Russians' intentions are in Syria, in Iraq, and to try to see if there are mutually beneficial ways forward here," the source told Al-Monitor.