The collection of rebel groups fighting to depose President Bashar al-Assad of Syria pushed further south on Friday toward a major city en route to the capital, as the government’s chief patron, Iran, moved to evacuate military commanders and other personnel from the country.

The rebels’ stunningly rapid gains spread alarm to neighboring countries, prompting border closures to guard against the prospect of further chaos as Mr. al-Assad’s authoritarian government lost more of its grip over swaths of the country.

And in another sign of the government’s loosening control, a Kurdish-led force backed by the United States, which is separate from the rebels advancing on Homs, said it had deployed in the eastern city of Deir al-Zour, which the government had previously held.

Taken together, the battlefield gains present the most serious challenge in years to Mr. al-Assad’s power.

But perhaps most significant was the withdrawal of Iranian personnel after more than a decade of staunch support for Mr. al-Assad. Those evacuated included top commanders of Iran’s powerful Quds Forces, the external branch of the Revolutionary Guards Corps, according to Iranian and regional officials.

Evacuations were ordered at the Iranian Embassy in Damascus, and at bases of the Revolutionary Guards, Iranian and regional officials said. Iranians began to leave Syria early Friday, the officials said, heading toward Lebanon and Iraq.

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