ISTANBUL — Arab states in the Persian Gulf have agreed to provide a monthly stipend of several million dollars to pay a “salary” to opposition fighters in Syria and encourage more defections from President Bashar al-Assad’s army, participants at an international conference on the Syrian crisis said Sunday.

The money is the first formal international support for the rebels, and officials attending the Friends of Syria conference said the substantial funds would also likely be used to purchase weapons on the black market for the rebel Free Syria Army.

Secretary of State Hillary Rodham Clinton also announced that the United States would contribute an additional $12.2 million for humanitarian aid to Syria, bringing the U.S. total since the uprising began to $25 million, as well as “communications equipment” to “help activists organize, evade attacks by the regime, and connect to the outside world.”

Foreign ministers and other top officials from more than 80 nations and international organizations gathered here for a Friends of Syria meeting publicly warned Assad that he has little time left to comply with demands he stop his year-long slaughter of his own citizens. In private, the officials debated what their governments will do if, as expected, Assad does not stop.

“The window of opportunity … is not open-ended,” the group said in a statement issued at the close of the meeting. It suggested “a return to the United Nations Security Council, if the killing continues.”

Despite Assad’s agreement more than a week ago to order his troops to stop attacking civilians, to allow the safe passage of humanitarian aid and to begin negotiations for a government transition, the statement said, the regime’s abuses “continue unabated.”

Kofi Annan, the U.N. and Arab League envoy who met with Assad, is due to deliver a status report to the Security Council on Monday, and the statement directed him to “determine a timeline for next steps … if the killing continues.” Influential participants in the Friends group indicated that the timeline would constitute a deadline, perhaps a matter of weeks, for Assad’s compliance.