The United Nations security council unanimously approved a Syrian peace plan on Friday, Dec. 18 in New York. (U.S. Department of State)

The U.N. ­Security Council on Friday unanimously approved a resolution endorsing a peace process that is designed to end Syria’s civil war and to allow the international community to focus its attention more fully on defeating the Islamic State.

“This council is sending a clear message to all concerned that the time is now to stop the killing in Syria and to lay the groundwork for a government that the long-suffering people of that battered land can support,” Secretary of State John F. Kerry said of the initiative.

The resolution gave Security Council backing to a process that begins with negotiations between the Syrian government and its opponents to establish a transitional government that will write a new constitution and hold elections, all within 18 months. It designated the United Nations to shepherd the process.

But the agreement made no mention of the future of Syrian President Bashar al-Assad, and it remains unclear to what extent the will of the international community can be imposed on him or rebel forces.

Kerry nonetheless described the agreement as a major step toward resolving Syria’s civil conflict, a goal that has eluded the countries backing the opposing forces on the ground for more than four years. The resulting chaos has allowed the Islamic State to occupy wide swaths of Syria and neighboring Iraq

U.S. Secretary of State John F Kerry and Foreign Minister of Russia Sergey Lavrov are seated next to each other before a meeting of Foreign Ministers about the situation in Syria at the Palace Hotel in New York Friday. (Carlo Allegri/Reuters)

For the first time, he said, a political track has been agreed to by Russia and Iran — both backers of Assad — and the United States and its partners in Europe and the Middle East who support the Syrian opposition trying to overthrow him. There is, Kerry said, a “clarity about the steps that need to be taken” and “a time frame.”

“Nobody is sitting here today suggesting that the road ahead is a gilded path,” Kerry said at a news conference after the vote.

The resolution instructs U.N. envoy Staffan de Mistura to guide the government and opposition groups toward talks to begin as early as next month.

Among the pitfalls he will have to navigate are the opposition’s insistence that no talks can be held with Assad’s participation and Russia’s demand that Assad be included. The two sides and their international supporters also disagree about which opposition groups can take part in the negotiations, with Russia seeking to ban as “terrorists” Islamist forces who are among the strongest and most numerous opposition fighters.

“I’m not too optimistic about what has been achieved today,” said Russian Foreign Minister Sergei Lavrov, who sat at Kerry’s side during the news conference. “The progress has not been as dramatic as we would like it to be. . . . But it is complicated.”

Kerry indicated that passage of the resolution, assuming its provisions begin to be implemented, could lead to more coordination between the United States and Russia on airstrikes against the Islamic State. “Now that we have a U.N. resolution . . . and a process moving,” he said, “the door is much more open . . . for us to consider greater ways of cooperation.”

The Security Council vote came after more than five hours of separate talks Friday among a group of more than a dozen nations, the International Syria Support Group, to hammer out the terms. First convened by Kerry two months ago, and including countries backing all sides of the conflict, the group has been propelled by shared concern over the growing humanitarian crisis in Syria, refu­gee flows to Europe, and anxiety over the growing strength of the Islamic State, including recent terrorist attacks it has sponsored or inspired in Europe, the United States and elsewhere.

But while the group agrees that a political solution has to be found, it has faltered on the terms. The resolution appeared to paper over many of the members’ differences in the interests of starting a process that Kerry has said he hopes will take on its own momentum and propel compromise among the disparate Syrian and foreign actors fueling the war.

To keep Russia and Iran in the tent, the United States has diluted its rhetoric on Assad and emphasized shared concerns about terrorism. To convince Saudi Arabia, Turkey and others that it recognizes the broad opposition to Assad inside Syria, the Obama administration has quietly dropped its refusal to recognize the Islamist groups they back as legitimate actors in a negotiated solution.

Those positions have required significant juggling on Kerry’s part.

After meeting in Moscow with Russian President Vladimir Putin this week, the secretary of state said: “While we don’t see eye to eye on every single aspect of Syria, we certainly agreed . . . that we see Syria fundamentally very similarly. We want the same outcomes. We see the same dangers. We understand the same challenges.”

While not seeking “regime change,” Kerry said, the United States and its partners “don’t believe that Assad himself has the ability to be able to lead the future Syria.” Some regional allies — and U.S. critics of the administration’s Syria policy — saw his statement as caving in to Russian demands by stepping back from insistence that Assad leave office.

Administration officials insisted that was not the case, saying that Kerry is merely setting aside the question of Assad to be taken up later in negotiations among Syrians. Kerry said he hopes those talks can begin as early as next month.

Putin, in a news conference Thursday in Moscow, said that his plan for Syria coincided in “key aspects” with U.S. goals: “working on the constitution, preparing elections in Syria and the recognition of their results.”

For now, he said, Russian airstrikes, most of which have focused on what Moscow says are opposition “terrorists” rather than the Islamic State, would continue.

In a meeting last month in Vienna, the International Syria Support Group agreed it would try to find common ground on which groups fighting in Syria — apart from the Islamic State and Jabhat al-Nusra, an al-Qaeda affiliate — could participate in talks with government representatives and which would be barred as “terrorists.”

The Assad government has submitted a list of more than three dozen representatives to participate in transition talks. In a meeting last week in Riyadh, the Saudi capital, representatives from a wide variety of opposition groups appointed a committee to choose negotiators.

Included in the Riyadh group were Islamist fighting groups such as Ahrar al-Sham, which the United States had previously indicated it considered extreme and would not support as part of the process. In a change in policy, administration officials said that eligible groups would be judged by their willingness to participate in a cease-fire.

Russia, presumably speaking for Assad, does not want Islamist groups to participate at all and has called for them to be deemed ineligible “terrorists.” That would allow Russia to continue bombing the groups, along with the Islamic State and Jabhat ­al-Nusra, under the terms of a cease-fire.

Iranian Foreign Minister Mohammad Javad Zarif, who attended the morning meeting in New York, accused “outside actors” of trying to impose “preconditions” on the Syrian people. In an opinion column in the British newspaper the Guardian on Friday, he said these actors were trying to differentiate between “good terrorists” and “bad terrorists.”

De Mistura said he would issue invitations in January to a first round of talks among Syrians but did not indicate who would be on the list. He warned against being “too ambitious” in terms of timing.