The Local Coordination Committees, a group of activists in Syria who organize and document demonstrations, said that most of the confirmed killings were in Binnish, in the northern province of Idlib, when troops stormed the town in search of dissidents who have challenged the authority of President Bashar al-Assad.

Residents said that security forces also shot at protesters who were staging a sit-in at the entrance of the town in an effort to prevent tanks and armored vehicles from entering it. They said that at least nine people were wounded in the shooting, four of them seriously.

Activists also said that troops destroyed the homes of four fugitive demonstrators who had played a leading role in organizing protests and that security forces were carrying out house-to-house raids to find them.

“The situation is very bad in Binnish,” said an activist reached by telephone from a nearby town. “Gunfire was heard throughout the day and we heard that a lot of people were dead and injured.”

Activists also said that one person was shot dead by army troops as he was walking along a street in Homs, a restive city in western Syria that has been an epicenter of some of the largest protests since the uprising against Mr. Assad began in mid-March.

Residents reached by telephone warned of a severe humanitarian crisis in several neighborhoods in the city, especially in areas under siege, where army defectors and deserters are believed to be hiding and where clashes between them and the army have been reported.

“The humanitarian situation is miserable,” said Abu Abshi, a resident of Homs reached by telephone. “We are facing a shortage in almost everything. Life is getting harder and harder by the day.”

Clashes were also reported in the southern town of Dara’a between army troops and defectors, though there were conflicting reports on casualties. The Syrian Observatory for Human Rights, a Syria activist group based in Britain, said that six soldiers, two army defectors and a civilian were killed in the fighting. Other activists said they could not confirm any deaths.

The United Nations said in a report published this month that at least 2,900 people had been killed in Mr. Assad’s repression of the uprising against him, which began in Dara’a seven months ago and spread across the country.

The Syrian government, however, disputes the death toll and has portrayed the uprising as a foreign conspiracy to divide the country. The government places blame for the unrest on armed foreign groups that it said had killed more than 1,100 soldiers and police officers.

The violence on Thursday came as the Gulf Cooperation Council, which includes Saudi Arabia, the United Arab Emirates, Qatar, Kuwait, Bahrain and Oman, called for an immediate meeting of Arab League states to discuss the “dire” situation in Syria and ways “to stop the bloodshed and machine of violence.”