President Bashar al-Assad’s political survival was under threat as his government battled opposition rebellions around the country on Saturday and his key ally, Iran, pared back military support.

The main rebel offensive that began on Nov. 27 had reached the outskirts of the strategic city of Homs by Saturday morning, only about 100 miles from the seat of Mr. al-Assad’s power in the capital, Damascus. But the opposition fighters were facing some of the stiffest resistance they have encountered so far from government forces there, who are trying to block the rebel advance moving rapidly in the direction of Damascus.

Syrian government forces are still stationed on the outskirts of Homs and were shelling areas newly captured by the rebels, according to a British-based war monitoring group, the Syrian Observatory for Human Rights. There were also clashes between rebels and government forces north of the city, the Observatory said.

Now, it is unclear how long Mr. al-Assad can hold onto the rapidly shrinking pocket of territory under his control, especially without the help of one of his staunchest allies, Iran. The regime in Tehran has lent robust military support that was crucial to Mr. al-Assad’s survival over the past 13 years of civil war.

But late on Friday, Iran moved to start evacuating military commanders and other personnel from Syria, according to Iranian and regional officials.

The main rebel offensive now approaching Homs is led by the Islamist group Hayat Tahrir al-Sham. It has taken major cities and large parts of four provinces since launching a surprise offensive last week out of their base in northwestern Syria.

In the southern province of Daraa — where the Syrian uprising against Mr. al-Assad’s authoritarian rule began in 2011 — a separate grouping of local rebel factions has taken control of more than 80 percent of the province after regime forces withdrew from checkpoints and military headquarters, according to the Observatory.

In the neighboring province of Suweida, local opposition groups attacked police and military checkpoints and took control of the main prison. The two rebellions in Suweida and Daraa both involved rebel groups that are separate from those in Homs.

The New York Times