More than 70 Western and Arab nations backing the Syrian uprising are meeting in Istanbul for talks aimed at further isolating President Bashar al-Assad, while urging the opposition to unify and offer a democratic alternative to his iron-fisted rule.

U.S. Secretary of State Hillary Clinton Sunday demanded Mr. Assad's forces halt operations targeting civilians or face “serious consequences.”

But the top U.S. diplomat acknowledged the United Nations' effort to broker an end to the violence is faltering as Mr. Assad ignores his pledge to abide by a peace plan proposed by U.N.-Arab League envoy Kofi Annan.

Clinton said Washington will offer another $12 million in humanitarian aid for Syria's embattled population, and also provide communications equipment to help opposition forces organize and evade government attacks.

President Assad, whose foreign ministry declared the revolt crushed, has said he accepts Mr. Annan's six-point proposal. But his comments have been treated with skepticism by Western and Arab governments as fighting on the ground continues.

Turkish Prime Minister Recep Tayyip Erdogan called on the international community to support the Syrian people's “right to self-defense” if the U.N. fails to stop President Assad's bloody crackdown on a year-long anti-government uprising.

China and Russia declined invitations to join Sunday's “Friends of Syria” conference. The two countries have twice protected the Assad government from censure by the U.N. Security Council, fearing such a step could lead to foreign military intervention. Iran, a close Syrian ally, was not invited.

Burhan Ghalioun, leader of the opposition Syrian National Council, called for the strengthening of Syrian rebel forces as well as “security corridors” inside the country.

Syria's political opposition, which remains divided, has not yet formally endorsed the Annan plan, and rebel fighters say they will not put down their arms until government troops and heavy weapons are withdrawn from populated areas.

On Saturday, Clinton attended a “strategic forum” between the United States and the six-nation Gulf Cooperation Council in Saudi Arabia to discuss bringing an end to Syria's crackdown and countering the growing threat of Iran.

The U.S. remains opposed to arming Syria's rebels, like some Gulf states have proposed, and is instead working to unify the country's splintered opposition and find ways to get humanitarian aid into the country.

Ahead of Sunday's meeting, Turkish Foreign Minister Ahmet Davutoglu said the Annan plan is the “minimum” of what Syria must do “urgently and without delay.”

Meanwhile, the London-based Syrian Observatory for Human Rights told VOA that government shelling and clashes between security forces and protesters left 25 people dead Saturday.

The United Nations says more than 9,000 people have been killed since unrest in Syria began more than a year ago.

In a letter to the U.N. Friday, Syria said acts by “armed terrorist groups” had led to the deaths of more than 6,100 people in Syria since the start of the uprising.