BEIRUT—Syrian President Bashar al-Assad and his inner circle are engaged in a high-stakes gamble for the future of their fractured nation, betting Russian attacks on rebel positions will shift momentum in the conflict and shore up support from their core constituency.

Russia’s intervention is lending credence to what is widely believed to be Mr. Assad’s ultimate aim: Leave only one opponent in the multisided war—Islamic State—and force the West to choose between the extremist group and his regime.

Jubilant Assad loyalists have boasted that Moscow’s expanded involvement has foiled more than four years of efforts by the West and its allies to dislodge the strongman by backing Syria’s more-moderate armed opposition. The U.S. and its Western allies have said Russian airstrikes are primarily targeting these rebels, and not Islamic State.

“The heroic and extraordinary move by our friends in the Russian Federation will create a new history and geography for the region,” Faisal al-Mekdad, Syria’s deputy foreign minister, said on state television late Wednesday. “This is a transitional period, not for us, but for those in the enemy camp. It is they who will make the shift,” he added, referring to the U.S. and its Arab allies.

Moscow is escalating its military role with airstrikes in direct support of a ground assault by pro-regime forces for the first time. On Thursday, these forces, including local and foreign militias overseen by Iran, expanded and intensified the offensive launched a day earlier, opposition activists said.

Syrian state media confirmed Moscow’s involvement in the offensive, which is targeting rebel-held enclaves in the countryside north of the central city of Hama. The Kremlin said Wednesday it was supporting the regime’s ground offensive.

Not all of Mr. Assad’s supporters view Russia’s combat role positively. Some within the regime, as well as political leaders in the region, warn it will only worsen sectarian tensions between Shiites and Sunnis across the Middle East and solidify a de facto partition of Syria between rebel groups and pro-regime forces.

The Shiite-linked Alawite sect is at the core of Mr. Assad’s regime and his support base, while Sunnis are the ones mostly fighting him. In the wider Middle East, Shiite Iran and its allies are competing for power and influence with Sunni states led by the Saudi monarchy.

An Alawite Syrian army officer warned Mr. Assad is on the cusp of ceding what was left of Syria’s sovereignty by welcoming a greater Russian role on top of an already robust Iranian one.

“Russia’s intervention won’t end the war,” he said. “It will divide the country.”

In such a scenario, the Alawites along with other minorities would be relegated to a western corridor connecting Damascus to the coast, as most Sunnis are pushed out to the rest of the country, where Islamic State and others will fight each other for control.

The rallying of Iranians and Russians to Syria’s depleted and hollowed out army represents a pivotal phase in the conflict.

Khattar Abou Diab, a professor of political science at the University of Paris, said Syrian troops and their foreign militia allies are unlikely ever to regain control of the whole country, turning Russia’s gambit into a dangerous test of wills with the West.

“If they destroy the moderate opposition, then they put everyone in front of two choices: Assad or ISIS,” he said, referring to Islamic State. “If this fails then plan ‘B’: Rule part of Syria and carve out the Alawite state.”

Defense ministers of the North Atlantic Treaty Organization, meeting in Brussels, said Russia’s actions will only augment the chaos in Syria.

The Russian military said it had carried out bombing raids against 27 targets around Syria overnight, hitting near the village of al-Safsafah where Syrian troops are fighting a mix of rebel groups. Opposition activists said Syrian and Russian forces bombarded the Ghab Plain, an agricultural area in the northwestern tip of Hama province, adjacent to the rebel stronghold of Idlib province. To the west is Latakia province, one of Mr. Assad’s coastal strongholds.

The contested area sits on Syria’s principal sectarian fault line between the Alawite minority and the Sunni majority.

The area around Hama is held by several rebel groups, including CIA-backed Free Syrian Army factions and extremist elements such as the Nusra Front and Ahrar al Sham, which have been collaborating in the effort against the Russian-backed offensive.

Mr. Assad’s forces have embarked on “a wide-scale attack aimed at uprooting terrorists’ gatherings and liberating the areas and towns which have been suffering from woes and crimes of terrorism,” said Ali Abdullah Ayoub, the chief of the general staff of the Syrian army.

Several thousand Sunnis, many of them previously displaced from other conflict zones in the country, were estimated to have fled one rebel-held town in the Hama countryside called Kfar Nabouda since the start of the blistering Russian-backed offensive, according to local opposition activists and citizen journalists. They are believed to have escaped to neighboring Idlib province, which is largely in the hands of Sunni Islamist rebel groups.

Russia on Wednesday conducted its first naval assault on opponents of the regime, firing a volley of missiles from the Caspian Sea. The U.S. said four of the cruise missiles landed in Iran, adding it wasn’t clear where in Iran they had landed. Russia’s defense ministry denied the report.

“All the rockets fired from ships struck their targets,” spokesman Maj. Gen. Igor Konashenkov said.

Tehran has largely kept Mr. Assad afloat so far by funneling billions of dollars to his regime and defending it with thousands of Shiite foreign fighters led by its main regional proxy the Lebanese Shiite militia Hezbollah.

But that didn’t stem the threats to Mr. Assad’s strongholds along Syria’s Mediterranean coast which were increasing before the Russian intervention.

Although many Alawites saw Russian President Vladimir Putin as their savior, some also fear his move is a dangerous prelude to hardened lines of control.

“Those who are rejoicing don’t realize that we have put all our eggs in one basket and that there is no point of return,” said an Alawite resident of the provincial capital of Latakia.

At the moment a mix of Islamist groups and fighters associated with a Western-backed rebel umbrella group known as the Free Syrian Army control territory south and north of Damascus. Islamic State dominates areas in the country’s eastern half.

Even within pro-regime areas, some see the potential for further division.

Moscow is expected to tighten its grip over Syria’s coast while Iran and Hezbollah dominate Damascus and areas bordering Lebanon—all to safeguard their own respective military and strategic interests.

In addition to a long-standing naval facility in Tartous, Russians have now established large military positions around Latakia.

“Russians are here for their own interests and there is a reason why they chose to expand their presence on the coast and focus on specific areas as part of the so-called war on terror,” said the Alawite officer. “I love my country and don’t want to see it under a new occupation in the name of protecting us and our lands and sanctities.”

While Mr. Putin insists he’s in Syria to fight “terrorism in all its forms” as part of a broader bid to promote a political solution to the conflict, many in the opposition and their supporters in the region say that any previous hopes that Moscow might lead diplomatic efforts for a negotiated settlement have been shattered by its military intervention on the side of Iran and Mr. Assad.

“This is a new version of the Shiite crescent under Russian patronage,” said Sami Nader, a Beirut-based academic and analyst, referring to Iran’s arc of influence in the Arab world.

Over the weekend, Mr. Assad sought to portray Russia’s military action in Syria as a service for the whole world both for combating terror and ending what he and his allies see as Washington’s hegemony and unilateralism.

“We have great faith in this alliance and the changes around the world,” he said in an interview with Iranian state media.