Syrian troops have advanced on a key rebel area in Homs, launching the most aggressive assault since the beginning of the military crackdown on the city.
Security officials boasted on state TV that the attack on Baba Amr in Homs would "clean" the area.
News agencies reported they could not get through to their contacts in Homs.
Activists trapped in the besieged city have used Twitter to detail the assault. Early reports suggested that the army was attempting to enter Baba Amr from the northeast.
A human rights watchdog and an activist told the AP that the troops had not got through.
"The army has started combing the area building by building and house by house. Now the troops are searching every basement and tunnel for arms and terrorists.
There remain only few pockets of resistance, said the anonymous source.
Homs-based activist Hadi Abdullah said that as the fighting intensified activists were evacuating families
"Shelling has been targeting places that were considered safe in the past," Abdullah said.
The Free Syrian Army said: " We have caused huge losses in the 4th division Assad army, from their weaponry to their actual lives. The Assad army did not enter Baba Amr at all and they will never enter Baba Amr, God willing, with the brave Free Syrian Armies' protection.
"The clashes continue nearby in an area called Alhakora. The regime keeps trying to [claim] we are tired, exhausted - these are lies. We will continue."
One hundred thousand residents are trapped in central Homs, according to other reports.
Three foreign journalists, Edith Bouvier, William Daniels and Javier Espinosa, are still thought to be in the city.