Syrian security forces are continuing their crackdown on anti-government protests across the country, with army tanks shelling the third city of Homs.
Several reports say the residential district of Bab Amro came under attack in the early hours of the morning.
Towns around Deraa in the south have been raided and a western suburb of the capital Damascus has been cut off.
Thousands have reportedly been arrested and hundreds killed in the crackdown.
The Syrian government insists it is pursuing "armed terrorist gangs".
It says it has seized arms and ammunition as well as 150 motorbikes it says the "terrorists" were using to launch attacks.
Meanwhile, diplomats at the United Nations say international pressure following the crackdown has caused Syria to drop its plans to run for a seat on the UN Human Rights Council.
There has been no official confirmation of the move.
Under siege
The BBC's Jim Muir in Beirut, the capital of neighbouring Lebanon, says that despite the crackdown, solidarity demonstrations are being reported from many parts of the country.
It seems that as soon as the flames are stifled in one area, they break out somewhere else, he adds.
One resident in Homs told the BBC the district has been under siege for four days, with no water, electricity or access to medical care.
He said there had been clashes between security forces and residents, who resisted troops with hunting rifles.
It has not been possible to verify the account.
Reports said heavy shelling began in the Bab Amro district at about 0530 (0230 GMT), and that hundreds of troops were moving into the area.
"Homs is shaking with the sound of explosions from tank shelling and heavy machineguns," Najati Tayrara, a human rights campaigner in the city, told Reuters news agency.
There are reports of theft and looting, and that the main shopping centre in the area has been badly damaged by bombing.
Situation 'normal'
In the town of Jassem, north of Deraa where the unrest began in March, mass demonstrations continued into the night even as the troops and tanks started to move in.
Jassem and other towns in the area have been surrounded by security forces for several days, declaring their defiance through frequent peaceful protests.
Nearby Deraa has been cut off by troops and tanks for over two weeks, with dozens killed and hundreds arrested.
The government says the situation there is now normal, but it has not allowed UN relief missions in.
The Syrian Observatory for Human Rights says 621 civilians and 120 security personnel have been killed since demonstrations pro-democracy protests began in March. Another Syrian rights group, Sawasiah, says more than 800 civilians have been killed.
Officials dispute the civilian toll and say about 100 soldiers have died.
Foreign journalists have not been allowed to enter Syria, so reports from the country are difficult to verify independently.
