The Syrian army is advancing on opposition positions in Homs, which has been under artillery bombardment for nearly a month, reports say.
Security officials in Damascus said the besieged district of Baba Amr would be "cleaned" within the next few hours.
Residents of Homs have long said they believe a ground assault is imminent, but the scale of the operation remains unclear.
It comes amid reports of a new draft UN resolution on the crisis.
The draft, which is still in its early stages, focuses on humanitarian aid for Syria, in the hope both China and Russia, which have opposed previous votes, will back it.
Although diplomats said the move was in the early stage, China said it supported "creating conditions" for humanitarian aid to be sent.
A UN meeting on Tuesday was told more than 7,500 had died in the 11 months of protest against Syrian President Bashar al-Assad.