A group of mostly exiled Syrian opposition figures on Thursday called on Syrian President Bashar al-Assad to resign, and U.S. Secretary of State Hillary Rodham Clinton hinted that the United States is preparing to take a tougher stance against Damascus, escalating pressure on the Syrian government as it presses ahead with its brutal crackdown against protesters.

In Washington, Clinton came closer than ever before to calling for Assad’s ouster and indicated that only China and Russia were preventing stronger action by the United Nations against the regime.

“The legitimacy that is necessary for anyone to expect change to occur under this current government is, if not gone, nearly run out,” she told a news conference.

The comment coincided with the conclusion of a two-day conference of 300 mostly exiled Syrian regime opponents in the Turkish beach resort of Antalya that was aimed in large part at persuading the international community to call for Assad’s departure.

A statement adopted by the delegates called on Assad to hand over power “immediately” to one of his deputies in order to pave the way for the creation of a transitional council, which would oversee the drafting of a new constitution before democratic elections within a year. A clause promising Assad immunity from prosecution if he did so, proposed by the Muslim Brotherhood, was removed at the last minute by a consensus of other delegates.

The statement also called for the creation of a democratic, secular Syrian state, in which freedom of worship would be guaranteed, but religion would play no role, and the rights of the country’s minorities would be respected.

The document signaled the beginning of an effort to form a coherent alternative to the government after nearly three months of spontaneous and mostly leaderless street protests. The protests have demonstrated the existence of widespread opposition to Assad but have not made clear what might come next if he falls.

Though the disparate assortment of aging exiles, Kurdish clergymen, bearded Islamists and youth activists cannot claim to represent the young street protesters who have braved bullets and tanks to spearhead the biggest challenge to Baath Party rule in decades, the conference was the first time oppositionists sought to unite around a single vision for Syria.

That they did so in just 48 hours without any significant quarrels, walkouts or dissent was hailed as remarkable by the participants. Youth activists, whose support was considered crucial if the effort was to have any credibility, welcomed the outcome.

“This marks the first time there has been a meeting of the opposition that completely represents all groups and voices,” said Yusuf Ramawi, who formed one of the first opposition Facebook pages nearly two years ago. “This is a big success.”

The participants also voted for the creation of a 31-member committee to represent the opposition in dealings with the international community and provide logistical support to protesters inside Syria. Its composition was carefully calculated to reflect Syria’s diversity, including four Kurds, four members of the Muslim Brotherhood, three women, three Alawites, a Christian and a Druze.

Syrian activists hope the committee will focus on pressuring the international community to toughen its response to the brutality in Syria, said Ahmad Raad, one of the founders of the Syrian Revolution Facebook page, which plays a leading role in coordinating the protest movement.

“The Syrian people on the inside see the international community as basically letting them down,” said Raad, who has been posting news of the conference on the page for Syrians inside the country to read.

Assad has shown no sign that he is prepared to relax the bloody crackdown, despite mounting pressure. Human rights groups said 15 people were killed Thursday as tanks continued to pound the besieged town of Rastan in central Syria, the latest focus of the effort to suppress the uprising.

But there were signs that he has been rattled by the unprecedented gathering of his opponents in neighboring Turkey, considered until recently one of Syria’s closest allies.

About 100 Assad supporters, who had apparently been bused in from Syria, staged a noisy demonstration outside the hotel where the conference was taking place, chanting slogans in support of the Syrian president and waving his picture as Turkish security forces kept them a safe distance away.

Staff writer Joby Warrick in Washington contributed to this report.