WASHINGTON — Syrian President Bashar Assad is now using most of his regular ground forces in an intensified drive to crush the uprising against his family's four-decade-long rule in what could be a critical test of his minority-run military's cohesion, according to U.S. officials and experts.

Assad and his inner circle are apparently betting that their ferocious artillery-backed onslaught on the city of Homs will deal a crippling blow to the insurgency, which will quickly be followed by seizures of other rebellious cities and towns. Refugees arriving in Lebanon say Assad's forces already have retaken the town of Zabadani.

The regime, however, may be in a race against time: The longer the insurgents hold out, the greater the risk of the army becoming overstretched; the more civilians killed and maimed in the wanton pummeling of opposition strongholds, the greater the threat of the army fraying, plunging the country into all-out sectarian war, some experts warn.

"We've been building up to a critical test over the last few months and that test is in the form of whether or not the regime can make significant gains in terms of taking back critical terrain," said Aram Nerguizian of the Center for Strategic and International Studies, a think tank based in Washington.

"This is increasingly becoming a sectarian war," said Joshua Landis, the director of Middle East studies at the University of Oklahoma.

The regime began intensifying military operations after the withdrawal last month of an Arab League observer mission, experts said. The mission left in disarray after failing to curb the bloodshed that was ignited when Assad's troops responded with gunfire against peaceful protests demanding his ouster that began just over a year ago in an outgrowth of the Arab Spring.

Assad was further encouraged to step up his scorched-earth strategy by Russia and China's Feb. 4 vetoes of a U.N. Security Council resolution that would have backed an Arab League peace plan calling for Assad to step down, U.S. officials and experts said. Two days later, the Obama administration released satellite photos of artillery and armored vehicles deployed around a smoke-shrouded Homs, home to an estimated 800,000 people.

Moreover, the United States, its NATO allies and Arab powers have ruled out military intervention and — at least for now — arming opposition fighters, while Russia and Assad's closest Middle Eastern ally, Iran, have continued to supply weapons to his army.

Testifying on Feb. 16 before the Senate Armed Services Committee, Director of National Intelligence James Clapper said that the Assad regime is now using "about 80 percent" of his 220,000-strong army's ground forces "in assaults on the civilian population."

An estimated 80 percent of the army comprises conscripts from Syria's Sunni Muslim majority, while about 60 percent of the officer corps is from the Assad family's minority Alawite sect, an offshoot of the Shiite branch of Islam that controls the regime's top posts.

A steady stream of deserters has been joining the Free Syrian Army, the name of the disparate bands of anti-Assad insurgents. But most of those deserters appear to be coming from reserve or non-combat units, not from Assad's main forces, Nerguizian said. They rely on smuggled and stolen arms, control only slivers of territory, have no coherent command structure and are waging largely defensive battles.