Secretary of State John F. Kerry, right, speaks with Turkish Foreign Minister Ahmet Davutoglu, left, at the U.S. Ambassador’s residence in Paris. (Pablo Martinez Monsivais/AP)

International backers of Syria’s fractious opposition warned rebel political leaders Sunday that upcoming negotiations for a transition government to replace President Bashar al-Assad may be their last, best hope to bring their country’s civil war to an end.

With less than two weeks to go before a negotiating conference in Geneva sponsored by the United Nations, the U.S.-backed Syrian Opposition Coalition has yet to say that it will attend. Amid rising frustration among its supporters, the group’s attempts to come to agreement and appoint a delegation have repeatedly broken down.

Coalition president Ahmad al-Jarba was recently reelected in a race that further split the political opposition. In meetings here with delegates, including Secretary of State John F. Kerry and foreign ministers from 10 European and Arab countries, Jarga said some opposition leaders remain suspicious that the United States and others can be depended upon to force Assad from power through a political agreement.

Speaking after the meeting, French Foreign Minister Laurent Fabius emphasized that the donors’ group was committed to “putting an end to the present [Assad] regime.”

In a formal statement, the countries known as the London 11, for the city where its delegates first met in the early days of the Syrian civil war, said that “as long as . . . Assad remains in power, there is no prospect of peace and stability in Syria and the region.”

Jarba is expected to relay the statement to the Syrian Opposition Coalition, which will meet Friday to consider attendance.

“I’m confident, personally . . . that the Syrian opposition will come to Geneva,” Kerry said in a news conference with Qatari Foreign Minister Khalid bin Mohamm­ad al-Attiyah. He said the opposition “understands the stakes.”

The Geneva conference, scheduled for Jan. 22, is to implement an agreement reached 18 months ago by a larger group of outside countries — including Russia, one of Assad’s main backers — calling for a transition government to be established in Syria through a negotiated settlement between the warring parties.

Conference delegations are to be chosen by “mutual consent,” which the opposition and its backers have said means Assad can neither attend nor remain in office. Assad, Russia and Iran, the Syrian president’s other principal supporter, have said that barring him from future power is an unacceptable “precondition” to the talks, although the government has agreed to attend.

The diplomacy came as fighting continued to rage between rebels and fighters with the al-Qaeda-affiliated Islamic State of Iraq and Syria, or ISIS, across most opposition-held areas of northern Syria.

At least 697 people have been killed in the 10 days since the clashes erupted between the two factions, according to figures published Sunday by the Britain-based Syrian Observatory for Human Rights. Most of the dead were said to be combatants, but at least 100 were civilians, including 21 activists who had been imprisoned by ISIS and were executed in Aleppo before the group lost control of its headquarters there.

The bodies of 11 people who had been handcuffed and shot in the head were uncovered Sunday at another ISIS base captured by rebels in Aleppo. The Islamist Ahrar al-Sham, which has played a prominent role in the revolt against ISIS, said on Twitter that as many as 100 of its fighters were killed Sunday in the capital of north-central Raqqah province.

Dozens of people were reported killed in attacks by government forces on rebel-held areas across the country, including 21 who died when airplanes dropped barrels packed with explosives onto the town of al-Bab in Aleppo province.

Kerry said the United States was moving toward a resumption of nonlethal military assistance to moderate opposition forces in northern Syria. The aid was suspended late last year after ISIS fighters took over a warehouse on the Turkish border where aid to U.S.-backed rebel groups was stored.

On Wednesday, Kerry will attend a conference of humanitarian donors to Syria. He said the war has displaced 8 million people and forced more than 2 million people to flee the country.

Although the Obama administration is expected to pledge more money, Kerry said that “the best solution to the humanitarian crisis is to get a political solution and end the creation of more refugees. . . . We are not looking for a policy of simply increased assistance to refugees. We are looking for a policy that saves Syria.”

Sly reported from Beirut.