Iran is sending commanders from its elite Iranian Revolutionary Guard Corps and hundreds of foot soldiers to Syria, according to current and former members of the corps.

The personnel moves come on top of what these people say are Tehran’s stepped-up efforts to aid the military of Syrian President Bashar al-Assad with cash and arms. That would indicate that regional capitals are being drawn deeper into Syria’s conflict—and undergird a growing perception among Mr. Assad’s opponents that the regime’s military is increasingly strained.

A commander of the Iranian Revolutionary Guard Corps, or IRGC, appeared to offer Iran’s first open acknowledgment of its military involvement in Syria.

“Today we are involved in fighting every aspect of a war, a military one in Syria and a cultural one as well,” Gen. Salar Abnoush, commander of IRGC’s Saheb al-Amr unit, told volunteer trainees in a speech Monday. The comments, reported by the Daneshjoo news agency, which is run by regime-aligned students, couldn’t be independently verified. Top Iranian officials had previously said the country isn’t involved in the conflict.

Iran has long trained members of the Syrian security apparatus in cybersecurity and spying on dissidents, U.S. officials and Syrian opposition members have said. The decision to send Iranian personnel comes after rebel attacks this summer in Syria’s biggest cities, Damascus and Aleppo, in particular an explosion in July that killed four members of Mr. Assad’s inner circle, according to the people familiar with the IRGC.

Syria’s regime is increasingly relying on a core of loyalists to conduct operations, say Syrian opposition members and rebel fighters. In recent weeks, Mr. Assad’s army has been hobbled by defections, losing territory in Kurdish areas as well as near Turkey’s border, these people say. On Monday, a Syrian military helicopter crashed in a ball of fire in Damascus, according to the Associated Press, citing activists and video footage.

Syria’s uprising has placed Iran in a foreign-policy predicament. As the Arab Spring unfolded in countries including Libya, Egypt and Bahrain, the Islamic Republic cast its own revolution as an inspiration for the uprisings.

But Tehran didn’t support the protesters in Syria—its closest ally in the region, the conduit between it and the Lebanese Shiite militant and political group Hezbollah, and a gateway for Iranian influence in the Arab world. Iran’s most influential voices, including its supreme leader and the political and military power structures, have steadfastly supported Syria’s president and, like Mr. Assad, have blamed the country’s violence on foreign meddling and terrorists.

But in continuing to support Mr. Assad, Tehran’s popular support in the region appears to have waned. Some elements of the government appear to be hedging bets: In the past few months, Iran’s Foreign Ministry has reached out to some Syrian opposition members, offering to mediate between the two sides.

Those efforts appear to be overshadowed now by Iran’s support for the Syrian military in its fight against the rebel insurgency, according to analysts and the former and current guard members.

“One of Iran’s wings will be broken if Assad falls. They are now using all their contacts from Iraq to Lebanon to keep him power,” Mohsen Sazegara, a founding IRGC member who now opposes the Iranian regime and lives in exile in the U.S., said by telephone.

On Thursday, Iran’s defense minister publicly signaled a shift. If Syria fails to put down the uprising, Iran would send military help based on a mutual defense agreement between the two countries, two Iranian newspapers quoted Defense Minister Ahmad Vahidi as saying. Syria hadn’t asked for assistance yet, he added.

“Syria is managing this situation very well on its own,” he said. “But if the government can’t resolve the crisis on its own, then based on their request we will fulfill our mutual defense-security pact.”

Syria’s crisis tops the agenda at the summit of Non-Aligned Movement nations this week in Tehran. Iranian Foreign Minister Ali Akbar Salehi said Thursday that Iran would announce a surprise peace plan for Syria during the five-day conference, which started Sunday.

In Tehran, Syrian National Reconciliation Minister Ali Haidar met Monday with several Iranian officials and expressed Syria’s gratitude. “The people of Syria will never forget the support of Iran during these difficult times,” Mr. Haidar said, according to Iranian media.

Iranian Supreme Leader Ayatollah Ali Khamenei, who has the final word in all state matters, has appointed Qasim Solaimani, the commander of the elite Quds Forces, to spearhead military cooperation with Mr. Assad and his forces, according to an IRGC member in Tehran with knowledge about deployments to Syria.

The Quds Forces are the IRGC’s operatives outside Iran, responsible for training proxy militants and exporting the revolution’s ideology. The U.S. blames the Quds Forces for terrorist attacks in Afghanistan and Iraq.

“Solaimani has convinced Mr. Khamenei that Iran’s borders extend beyond geographic frontiers, and fighting for Syria is an integral part of keeping the Shiite Crescent intact,” said the IRGC member in Tehran. The so-called Crescent, which came together after Saddam Hussein’s fall, includes Shiites from Iran, Iraq, Lebanon and Syria.

Iran is now sending hundreds of rank-and-file members of the IRGC and the basij—a plainclothes volunteer militia answering to the guards—to Damascus, said two people in the IRGC familiar with the movements.

Many of the Iranian troops hail from IRGC units outside Tehran, these people say, particularly from Iran’s Azerbaijan and Kurdistan regions where they have experience dealing with ethnic separatist movements. They are replacing low-ranking Syrian soldiers who have defected to the Syrian opposition, these people said, and mainly assume non-fighting roles such as guarding weapons caches and helping to run military bases.

Iran is also deploying IRGC commanders to guide Syrian forces in battle strategy and Quds commanders to help with military intelligence, Mr. Sazegara and the current IRGC members said.

On the other side of Syria’s conflict, Saudi Arabia and Qatar have funded and armed opposition rebels, while Turkey has allowed them to keep an unofficial base near Syria’s border. Foreign Arab fighters, many of them extremist Jihadist, have also flocked to Syria to fight alongside rebels.

Iran has also started moving military aid and cash to Syria through Iranian companies in Iraq, such as a construction company owned by a former IRGC member now living in Iraq and a tour company servicing pilgrims to holy Shiite sites, said Mr. Sazegara and a person in Iran familiar with the construction company.

The IRGC and Syrian forces are working together to free 48 Iranian hostages kidnapped by a unit of the opposition Free Syrian Army this month, according to two IRGC officials in Tehran as well as comments from an Iranian parliamentarian in Damascus this week.

Iran at first denied the kidnapped Iranians had any link with the IRGC. But Mr. Salehi later said some of the hostages were retired members of the IRGC, calling them Iran’s “most dear and beloved.” Iranian opposition media, meanwhile, have named four of the men, calling them current IRGC commanders from various Iranian provinces.

Iran’s ambassador to Syria said recently that the hostages’ whereabouts have been determined and that Iran is negotiation with Syria on how to rescue them, Iranian media reported. The envoy also said Iran and Syria had formed a joint committee, with intelligence, policy and military experts, for the rescue mission. Iranian media said Monday that this committee sends Mr. Assad regular updates of their findings.

WSJ