United Nations Secretary-General Ban Ki-moon is calling for a “coherent” international response to Syria's violent crackdown against dissent.

Mr. Ban on Thursday 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.

Earlier 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.

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

Separately Thursday, Syria's state news agency aired what it called the “confessions” of Colonel Hussein al-Harmoush, a senior military officer who publicly defected two months ago and fled to Turkey. Activists say Syrian intelligence agents recently captured Harmoush in Turkey and brought him home.

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.