U.S. Secretary of State John Kerry says the upcoming multilateral talks on Syria will not find an immediate political solution but represent the best hope available.
Kerry was speaking at the Carnegie Endowment for International Peace in Washington on October 28, before departing for Vienna where he is to meet with his counterparts from a dozen countries to find consensus on a political way forward in Syria.
Assad's main regional backer, Iran, is to take part for the first time in such a multilateral meeting on the four-year-long war, along with Damascus' other key ally Russia.
“The challenge that we face in Syria today is nothing less than to chart a course out of hell,” Kerry said in his speech.
"While finding a way forward on Syria will not be easy -- it's not going to be automatic -- it is the most promising opportunity for a political opening we have seen," he added.
Tehran has announced that its Foreign Minister Mohammad Javad Zarif will participate in the Vienna talks, which will begin with a meeting of the United States, Russia, Saudi Arabia, and Turkey on October 29.
The high-level talks will be expanded to include Iran, Iraq, Egypt, Lebanon, and others on the following day.
European Union foreign policy chief Federica Mogherini also confirmed she will attend.
Iran's Foreign Ministry spokeswoman Marzieh Afkham said three deputy foreign ministers will accompany Zarif at the talks.
Iranian news agencies quoted Afkham as saying the Iranian, Russian, and EU delegations will hold talks on the sidelines of the summit.
She also said the Iranian and U.S. delegations may also meet to discuss the implementation of the landmark nuclear deal Tehran signed with six world powers in July that curbs Iran's nuclear program in exchange for relief from economic sanctions.
Zarif and Russian Foreign Minister Sergei Lavrov spoke by telephone on October 28 to discuss the conflict in Syria and plans for the upcoming summit.
Kremlin spokesman Dmitry Peskov told reporters that Russia wanted a "widening of the dialogue" on Syria.
White House spokesman Eric Schultz said on October 28 that the United States is “prepared to work with any nation, including Russia and Iran, to resolve the conflict in Syria."
He added that the United States believes that for the multilateral talks to work "all key stakeholders to be present."
However, a spokesman for Syrian forces fighting against President Assad said Iran's participation in the Vienna talks could make it more difficult to reach a political solution to the crisis.
"Iran's presence will complicate the Vienna talks because it will come with a project to preserve Assad," Syrian National Coalition Vice President Hisham Marwa told Al-Arabiya television.
Two previous rounds of talks on Syria in 2012 and 2014 agreed on the principle of forming an interim unity government and sidelining Assad.
But Moscow, along with Tehran, insists that Assad must be part of any transition government, while the United States and its allies want the Syrian leader to step aside.
In his speech, Kerry said Russia and the United States both want "a united, secular Syria" in which citizens can choose their own leader through elections.
"We agree that all of these steps can only be achieved, and Syria can only be saved, through a political settlement," he said, arguing that "one man," Assad, cannot be allowed to stand in the way of peace.
Earlier, French Foreign Minister Laurent Fabius said France and its allies had agreed on the need for a "precise timetable" for Assad's departure.
But U.S. Deputy Secretary of State Tony Blinken said he didn't expect "any grand or final conclusions from the meetings in Vienna."
"It's just part of a process to see if we can get to an agreement on what a political transition should look like," he added.
In Riyadh, Saudi Foreign Minister Adel al-Jubeir said Saudi Arabia will take part in the Vienna talks to test the willingness of Russia and Iran for a political solution to the conflict.
Iran is supporting the Damascus regime through financial support and military advisors. Iranian officials have said that Tehran has in recent weeks boosted the presence of its military advisors in Syria.
Russia has also stepped up its military role in the Syrian war, launching a bombing campaign in support of Assad on September 30.
Russia said on October 28 that its warplanes had struck 118 "terrorist" targets in Syria over the previous 24 hours, attributing the rise to new surveillance data.
Both Russia and Iran insist that the Syrian leader must be part of any transition government.