Syrian opposition members announced Thursday the formation of a national council to present a unified front against President Bashar al-Assad.

Marking six months since the uprisings in Syria began, a group of opposition members gathered in Istanbul to introduce a council of 140 members, half living in Syria and the rest exiled dissidents.

A spokesman for the U.S. State Department expressed U.S. support for the group, but also noted the need for coordination as the council faces many challenges.

Meanwhile, the United States is urging Americans to immediately leave Syria, as it tightens its travel warning for the country.

United Nations Secretary-General Ban Ki-moon said Thursday that “enough is enough” on President al-Assad's broken promises for reform. He spoke as reports of violence continued to seep from the tightly sealed borders of Syria.

Also Thursday, the Syrian state news agency SANA aired the “confessions” of Colonel Hussein al-Harmoush, a senior military officer who publicly defected a couple months ago and fled to Turkey. Syrian activists say al-Harmoush recently was captured by Syrian intelligence in Turkey and brought back to Syria.

On Wednesday, activists say, Mr. Assad's security forces fired heavy machine guns during raids in northwestern villages, despite calls to end the violence. Forces opened fire on several villages in the Idlib province near the Turkish border, killing at least four people.

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

SANA reported Wednesday that Syrian ambassador to the Arab League Youssef Ahmed rejected the body's latest statement on the Syrian crisis as a “hostile and unconstructive act.”

Arab League foreign ministers met in Cairo Tuesday and called on Damascus to stop violence in the country immediately and launch a national dialogue. SANA said Syrian envoy Ahmed told the meeting that Damascus is moving ahead with a reform process.

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