The Syrian military is scrambling to block the rapid southward advance of antigovernment rebels by trying to cut off the road that connects the major cities of Hama and Homs, the opposition fighters and a war monitor said on Friday.

The Syrian rebels captured the central city of Hama on Thursday after government forces withdrew , prompting the military to try to slow the insurgents by erecting earth berms and carrying out airstrikes on a major highway to the south.

The forces of the Syrian president, Bashar al-Assad, also have withdrawn from other parts of the country that they control in the northeast and east, according to the monitor. They are reinforcing the most crucial areas for the survival of the Syrian government and its allies, like Homs; the capital, Damascus; and the coastal areas.

Homs lies at the heart of a critical land corridor that runs from Lebanon to Iran, which serves as a pipeline used by Tehran to send weapons, supplies and people to Hezbollah, an armed Lebanese group that is Syria’s most important regional ally.

Throughout the civil war, the Syrian regime has relied on Iran and Hezbollah — in addition to Russia — to fight the rebel efforts to oust it from power.

Despite the effort to stem their advance, the rebels on Friday continued their push south toward the city of Homs, about 30 miles south of Hama, and also captured some towns in the northern countryside, according to the rebels and the Syrian Observatory for Human Rights, a British-based monitor.