Syrian security forces shot and killed at least four people during a raid Friday as they continue to wage a violent campaign against protesters flooding the streets after prayers.

The London-based Syrian Observatory for Human Rights says 11 others were wounded earlier during the raid in the central province of Hama. Activists say there were no immediate details on the number of causalities or people wounded as forces fired on protesters outside mosques around the country.

On Thursday, United Nations Secretary-General Ban Ki-moon called for a “coherent” international response to Syria's violent crackdown against dissent, as the uprisings entered their seventh month.

Mr. Ban accused Syrian President Bashar al-Assad of “escalating violence and repression” and ignoring appeals to stop state-sponsored violence. The U.N. chief said “enough is enough” regarding Mr. Assad's broken promises of reform.

Also Thursday, Syrian opposition activists announced in Istanbul, Turkey, the formation of a council to present a united front against Mr. Assad. They said the council includes 140 members with half of them living in Syria.

The United States expressed its support, but noted the need for coordination as the group faces many challenges.

U.S. officials urged Americans to immediately leave Syria on Thursday, repeating their warning issued last month. Since then, Washington has imposed additional sanctions on the Syrian government.

The United Nations estimates that 2,600 people have been killed in Syria's six-month uprising against Mr. Assad's 11-year autocratic rule. The Syrian government said Monday about 1,400 have been killed, half of them security personnel.