Nine people were killed in shelling and gunfire by security forces in south-western Syria on Wednesday, a human rights group said, citing medical sources.
Avaaz said residents reported dozens of people injured in the province of Homs, while ambulances were being prevented from entering the streets of its Bab Amro neighbourhood, dpa reported.
More than 500 people have been arrested in the province since Monday, and were being held at a football stadium in the provincial capital, also called Homs.
Telephone lines to Homs and other parts of the country were down on Wednesday.
On the Syrian Revolution Facebook page, activists wrote that the sound of gunfire and shelling was heard throughout Homs, Syria's largest province.
Troops have maintained a presence in several cities amid ongoing anti-government protests, in which thousands have been calling for greater freedoms and the ouster of President Bashar al-Assad. Protests inspired by uprisings in Tunisia and Egypt have been taking place since mid-March.
The crackdown has drawn increasing international criticism. The United States on Wednesday called the Syrian government's actions "barbaric" and urged it to heed the wishes of its people.
"We believe their focus should be on meeting the legitimate aspirations of the Syrian people and not on how much violence or how many deaths it would take to force them into submission," State Department spokesman Mark Toner told reporters in Washington.
The website Syrian Martyrs has released the names of 757 people, including protesters, soldiers and security officers, its organizers say have been killed since the protests began.
On Sunday, 10 people were killed and three injured in Homs. The state-run news agency SANA said the group that had returned from working in Lebanon had been "ambushed by an armed terrorist group in Homs."
SANA also reported that three military officers and three soldiers were killed in security sweeps in Homs and other cities, including Daraa and Banias, on the same day.
On Wednesday, UN Secretary General Ban Ki-moon said he was "disappointed" that al-Assad has not allowed a UN commission access to Daraa and other cities, as promised, to investigate.