Iran usually tries to remain in the background in conflicts, preferring to support local groups rather than sending its own soldiers into battle, but the United States has often accused Iran of sending troops to Afghanistan and Iraq. After two and a half years of fighting, Mr. Assad’s military has begun to wear down, losing momentum and territory — prompting its allies in Iran, and Hezbollah in Lebanon, to provide more robust support.

With Iran’s help, Syria began to build up militias that took some of the pressure off the conventional forces.

During his speech to Guards commanders in Tehran on Monday, Mr. Rouhani praised the role of Iran’s Revolutionary Guards Corps in securing Iran’s interests in the region, though he did not address their having any active role in Syria. The West, Mr. Rouhani said, is executing a plan in the Middle East “to consolidate the power of Israel and weaken the resistance,” referring to its military and ideological alliance with Hezbollah and with Syria.

Syria is of great significance for Iran, he told the commanders. “We will not remain indifferent towards this critical issue.”

Mr. Rouhani said the West should realize that the Revolutionary Guards do not seek military dominance in the region. “Our discourse is the discourse of democracy, brotherhood and unity,” he said. “Our discourse is fighting with terrorism in the region.” (Mr. Assad has long called the rebels fighting him terrorists.)

In the footage released to the Dutch public broadcasting program by the Syrian rebel group, the Dawood brigade, one Iranian commander, who calls himself “Haj Esmail,” sits down for long, in-depth interviews with an unnamed videographer. He speaks of training Syrian militia members loyal to Mr. Assad in Tehran and criticizes the Syrian Army for being too rigid with its fighters. “They want to come to our fronts because we show them respect,” he said.

“Syria is a war between Islam and nonbelievers,” Mr. Haj Esmail said in one of the interviews. “Good versus evil, our front is supported by Iranian fighters, Hezbollah and mujahedeen fighters from Iraq and Syria,” he said. “Our opponents are Israel, Saudi Arabia, Turkey, Qatar, funded by the Emirates, and Britain, France and the United States.”