Air strikes targeted a bridge on the highway linking the Syrian cities of Homs and Hama, a war monitor said on Friday, as government forces scramble to secure Homs after Islamist-led rebels captured Hama and commercial hub Aleppo.

"Fighter jets executed several airstrikes, targeting Al-Rastan bridge on (the) Homs-Hama highway... as well as attacking positions around the bridge, attempting to cut off the road between Hama and Homs and secure Homs," the Britain-based Syrian Observatory for Human Rights said.

The rebels led by Islamist group Hayat Tahrir al-Sham (HTS) launched their offensive a little more than a week ago, just as a ceasefire in neighbouring Lebanon took hold between Israel and Syrian President Bashar al-Assad's ally Hezbollah.

To slow the rebel advance, the Observatory said Assad's forces erected soil barriers on the highway north of Homs, Syria's third-largest city which lies just 40 kilometres south of Hama.

Tens of thousands of members of Assad's Alawite minority community were fleeing Homs on Thursday, for fear that the rebels would keep up their advance, the Observatory said earlier.

The rebels captured Hama on Thursday following street battles with government forces, announcing "the complete liberation of the city" in a message on their Telegram channel.

Rebel fighters kissed the ground and let off volleys of celebratory gunfire as they entered Syria's fourth-largest city. (AFP)