Forces fighting for Syrian President Bashar al-Assad are struggling against rebels in the country’s northwest, despite substantial backing from Russia and Iran, according to U.S. officials. But they caution that both Russia and Tehran seem prepared to dig in and intensify their efforts.

“Pro-regime forces have had little success to date, despite a marked increase in Russian airstrikes in recent days,” a U.S. intelligence official told VOA on condition of anonymity.

Another U.S. official, who also was not authorized to discuss the Syrian offensive publicly, characterized the Russian and Iranian-backed offensive in northwest Syria as making only “incremental gains.”

“The [Syrian] opposition is definitely putting up a formidable fight in that area,” the official added.

In the meantime, Russian airstrikes on rebel positions are starting to take a toll.

Commanders with Fatah Halab [Conquest of Aleppo], which brings together more than 30 rebel factions U.S. officials and analysts also caution that the number of airstrikes targeting top rebel leaders is only likely to increase, as Russia expands its use of drones and other advanced intelligence gathering techniques.