But most of the airstrikes appear to be targeting anti-Assad Syrian groups, including one that has received weapons and training from the United States. The airstrikes are focused on the juncture of Idlib, Hama and Latakia provinces in Syria’s north, where rebel groups have gained ground this year. With Russian air support, the Syrian army (reportedly backed by paramilitary forces from Lebanon, Iraq and Afghanistan) launched an offensive last week in the northern province of Hama in an attempt to roll back recent rebel gains and safeguard the core government-controlled areas along the Mediterranean coast. On Saturday the Russian Defense Ministry said that warplanes carried out 67 combat missions in Syria — the highest daily tally since the airstrikes commenced on Sept. 30.

Still, the intensity of the Russian airstrikes and the evident determination of Putin to preserve the rule of his Syrian ally could compel ISIL to reconsider any plans it may have drawn up to extend its caliphate westward.

In May, ISIL captured the ancient town of Palmyra in central Syria. By early August, its fighters advanced 60 miles to the southwest to seize Qaryatayn. From there, ISIL could head west toward the central government corridor between Homs and Hama, move south toward Damascus or southwest into the mountainous Qalamoun region that straddles the border with Lebanon.

The key highway linking Damascus to Homs and the Mediterranean coast passes through Qalamoun. The Assad government has long been concerned that if rebel forces cut the highway, the capital would be effectively sealed off from the core government-controlled areas in the coastal mountains.

Underlining the importance of Qalamoun, Hezbollah, the Iranian-backed Shia group in Lebanon that has been a key ally of Assad’s, has spearheaded two operations since November 2013 to drive rebel forces out of the region’s towns and villages and the surrounding mountains. A few hundred rebels, mainly from ISIL and Jabhat Al-Nusra (the Nusra Front), Syria’s Al-Qaeda franchise, are still holed up in rugged mountains on the Lebanese side of the border near Ras Baalbek and the neighboring town of Arsal.

Even if ISIL abandons a push toward Lebanon from Qaryatayn, the few hundred fighters tucked into abandoned farms and quarries near Ras Baalbek still pose a threat. Barely a day goes by without an exchange of fire between the fighters and the Lebanese army, which mans a series of fortified outposts and checkpoints to block any incursions into populated areas of Lebanon. The nearest ISIL positions to Ras Baalbek are less than 3 miles to the east. Nasrallah and his men, with support from Hezbollah, man a number of observations posts, keeping a wary eye on the barren sepia-colored mountains that rise east of the village.

“We are hearing that Daesh [ISIL] has been able to bring in a few reinforcements lately,” said a Western intelligence source in Beirut.