The BBC reported activists said the forces of President Bashar Assad used airstrikes in combination with tanks and artillery against rebel positions. The British network said unverified video footage showed clouds of smoke from explosions across Homs.
Syrian state TV said the military experienced "great success" while "killing many terrorists."
The New York Times reported the offensive indicated Assad was pressing to consolidate control of areas other than the capital, Damascus.
The newspaper said Dr. Walid Fares, an activist in Homs, said the government had spent several days positioning troops before initiating strikes against the rebels.
"They used rocket launchers for hours," he said, adding government troops "shelled us with heavy mortar shells" and "raided us from four different directions."
He said power and telecommunications in sections of Homs had been cut.
"They've blocked everything here -- electricity, phones, water," he said. "The only thing they haven't blocked is the air we breathe."