Syrian army tanks have been shelling the third biggest city of Homs, as security forces continue their nationwide crackdown on weeks of anti-government protests.
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.
UN Secretary General Ban Ki-moon called on Syria to take a softer line.
"I urge again President [Bashar al-]Assad to heed calls for reform and freedom and to desist from excessive force and mass arrest of peaceful demonstrators," he told journalists.
He said he was disappointed that Syria had not yet allowed an international aid assessment team access to Deraa, where the unrest began in March, despite assurances from Mr Assad.
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 that Bab Amro district had 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.
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Reports said heavy shelling began in Bab Amro at about 0530 (0230 GMT) on Wednesday, and that hundreds of troops were moving into the area.
One eyewitness reported seeing three dead bodies in the centre of the district.
Another said there had been a "cautious calm" in the area since 0700, "only interrupted by occasional sounds of gunfire".
A third eyewitness told the BBC security in Homs was extremely tight.
"Always when we go on the streets, around our jobs and the city centre we find the tanks on the bridges," he said.
"They divided the city into three or four regions ... and inspect everybody who comes in. Nobody can go out.
"We see the tanks with [many] soldiers, fully armed, and we hear the sounds of firing from inside these regions. But we have no ability to go there to see what's happening or to give food or to give help to the injured people."
There are reports of theft and looting, and that the main shopping centre in the area has been badly damaged by bombing.
In the town of Jassem, north of Deraa, 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.