Anti-government activists report that 12 people have been killed so far on Tuesday and 100 injured. This cannot be independently confirmed.
Activists have been warning of reinforced government forces around Homs, ahead of a ground assault on the city.
Thousands have died there in an 11-month uprising against the government.
Human rights groups believe more than 7,000 people have been killed since the uprising began.
The Syrian government says at least 2,000 members of the security forces have died fighting militants.
Reports from the city say the field hospital has been hit, and pictures show tanks moving through the city.
Our correspondent says it is unclear if the bombardment is the precursor to a much-feared ground assault on the Baba Amr district of the city.