Syrian opposition activists have announced the formation of a council to present a united front against President Bashar al-Assad as United Nations chief Ban Ki-moon called for international action against Syria's bloody crackdown.

Marking six months since the Syrian uprising began, opposition members gathered in Istanbul Thursday said the council includes 140 members, half living in Syria.

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

Mr. Ban accused the Syrian leader 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. He spoke as reports of violence continued to seep from Syria's tightly sealed borders.

Meanwhile, the United States is urging Americans to immediately leave Syria. The U.S. repeated a warning issued in August, before additional U.S. sanctions were imposed on the Syrian government.

Also Thursday, Syria's state news agency aired 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 al-Harmoush in Turkey and brought him home.

On Wednesday, Mr. Assad's security forces opened fire on several villages in Idlib province near the Turkish border, killing at least four people.

The London-based Syrian Observatory for Human Rights says troops arrested 100 people, including family members of a soldier who defected.

Arab League foreign ministers met in Cairo Tuesday, calling on Damascus to stop violence and launch a national dialogue.

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.