Tanks have been shelling Syria's third biggest city, Homs, as security forces continue their crackdown on nationwide anti-government protests, reports say.
Activists said there had been gunfire and explosions in the district of Bab Amr and nearby villages, killing at least nine people and wounding dozens.
Several towns around the southern city of Deraa were also raided and at least 11 civilians killed, activists said.
Clashes were also reported in Syria's second-largest city of Aleppo.
Witnesses there said security forces used batons to disperse a protest by about 2,000 students at the city's university campus.
Thousands of people have reportedly been arrested and hundreds killed in the government crackdown.
The Syrian government insists it is pursuing "armed terrorist gangs", and blames them for the deaths of two soldiers on Wednesday.
Meanwhile, international pressure has caused Syria to drop its plans to run for a seat on the UN Human Rights Council. Kuwait, which was expected to stand in 2013, will take its place.
UN Secretary General Ban Ki-moon called on President Bashar al-Assad to "heed calls for reform and freedom and to desist from excessive force and mass arrest of peaceful demonstrators".
The BBC's Jim Muir in Beirut says that despite the crackdown, solidarity demonstrations are being reported in many parts of Syria.
One resident in Homs told the BBC that Bab Amr had been under siege since Saturday, with no water, electricity or access to medical care.
Control seemed to have been imposed by Tuesday and the latest flare-up came as a surprise, our correspondent adds.
Reports said the shelling began in Bab Amr at about 0530 (0230 GMT) on Wednesday, and that hundreds of troops had moved in.
"Homs is shaking with the sound of explosions from tank shelling and heavy machine-guns," human rights activist Najati Tayara told Reuters.
One resident told BBC Arabic that a prominent Christian businessman had been shot dead by a sniper, and claimed the security services had a "plan for sectarian division in Homs between Christians and Muslims".
"He was killed at the door of his house at 0600 this morning as he stepped out to inspect the sounds of tank shelling in Bab Amr and to protect his son," he said.
Another resident told the BBC that security in Homs was extremely tight.
Activists told the BBC that about 500 people had been arrested in Homs since Wednesday, including more than 100 on Tuesday night.
It has not been possible to verify the accounts because foreign journalists have not been allowed to enter Syria.
The state news agency, Sana, reported that troops and security agents had "arrested dozens of wanted men and seized large quantities of weapons and ammunition in Bab Amr", as well as in Deraa.
It cited sources as saying that one soldier was killed and four were injured in Bab Amr, while one was killed and another injured in rural Deraa. A number of "terrorists" were killed and injured, it added.
Sana also said the government had formed a commission to draft a new law on legislative elections within the next two weeks.
'Fight until the end'
In other developments on Wednesday:
- At least 11 people were killed when tanks shelled the southern town of al-Harra, activists told the BBC
- Troops and tanks move into the nearby town of Jassem overnight. Activists told the Associated Press that three protesters died
- Activists told Reuters that four civilians were killed by security forces and 300 were detained in Tafas, just north of Deraa
- The army continued its operation in the coastal town of Baniyas, where at least 300 people have been arrested since Saturday
- A leading member of the opposition People's Democratic Party, Mazen Adi, was detained in Damascus, while political activist Wael al-Qaq has been arrested on the Lebanese-Syrian border
- The UK Foreign Office described the level of repression as "shocking" and urged the Syrian government to change its behaviour
Deraa, where the unrest began in mid-March, has been cut off by troops for more than two weeks, with dozens killed and hundreds arrested.
The government says the situation there is now normal, but it has refused to allow UN humanitarian teams in.
The Syrian Observatory for Human Rights says 647 civilians have been killed since pro-democracy protests began on 18 March. Another rights group, Sawasiah, says more than 800 civilians have died.
Officials dispute the civilian toll and say about 100 soldiers have died.