An image of Syrian President Bashar al-Assad is seen on a car parked in front of damaged buildings in the town of Rabiya. (Omar Sanadiki/Reuters)

On the eve of Syrian peace talks scheduled to begin Friday in Geneva, it remained unclear whether the government or the opposition would show up, or what they would talk about if they do.

Opposition leaders meeting for the fourth day in Saudi Arabia debated whether they should enter negotiations without some indication that the government of Syrian President Bashar al-Assad and his Russian backers are ­prepared to halt bombardments of civilian areas or take other ­confidence-building steps.

As opposition leaders expressed growing doubt about whether the U.S-led international coalition that supports them is ready to press their demands, the United States voiced growing exasperation at their hesi­ta­tion.

“We believe that the opportunity that is presented by these talks should be sweetener enough for the HNC to come to the table and talk” without “preconditions,” State Department deputy spokesman Mark Toner said, referring to the High Negotiating Committee, the group formed to represent the opposition in Geneva.

In addition to concerns about confidence-building steps that the opposition says it believed would take place before the talks began — including the release of some prisoners and a lifting of government sieges that have prevented humanitarian aid from reaching numerous population areas, as well as an end to the bombing — the opposition is increasingly uneasy about what it sees as U.S. backtracking on Assad’s future.

[Russian airstrikes in Syria are generating gains for government forces]

In a resolution passed last month, the United Nations called for the negotiations “on a political transition process” to begin in January. Backed by Russia and the United States, the resolution also expressed support for a nationwide cease-fire to take effect as soon as initial transition steps had been taken.

In recent days, however, Secretary of State John F. Kerry and Staffan de Mistura, the U.N. envoy shepherding the talks, have indicated they want to begin negotiations with cease-fire talks, and have played down the urgency of a transition that would remove ­Assad from power.

Last weekend, Kerry denied opposition charges that the United States would settle for a “unity” government, in which both Assad and the opposition would take part, rather than the transition called for in the U.N. resolution and other preparatory documents.

The difference, he indicated, was one of semantics, with Russia backing “what they call a unity government, but everybody else calls a transitional government.”

“We support getting a cease-fire, we support getting humanitarian access. . . . We’ve said 100,000 times Assad cannot be part of the long-term future of Syria,” Kerry said. “It’s very simple. Nothing has changed.”

What has changed, however, are conditions on the ground. In the weeks since the talks were outlined, Assad forces, backed by Russian airstrikes, have made significant progress in retaking key areas of opposition control. The rebels “are being pulverized,” said Andrew Tabler of the Washington Institute for Near East Policy.

Hadi al-Bahra, an opposition spokesman in Istanbul, said the military advances suggest that neither Russia nor Syria is serious about peacemaking. “They are expending their military operations on the ground and using the political process to give the image that they are seeking a political solution, while in actuality . . . they are pushing for a military solution,” Bahra said.

Assad’s government has made little comment while the opposition, acutely conscious that it is likely to be blamed if the talks fail to start, tries to decide what to do.

As the scheduled Geneva start time approached, opposition leaders reached for outside support. On Tuesday, the HNC sent a letter to U.N. Secretary General Ban Ki-Moon asking for assurances that humanitarian measures would be implemented. Opposition representatives said the reply came from de Mistura, who said that it was up to U.N. Security Council members to ensure implementation of the resolution.

In a video message to the Syrian people Thursday, de Mistura urged them to press their representatives to attend. “You must know that we count on you to raise your voice to say . . . enough, to say to everyone who is actually coming, from Syria and abroad, to this conference that there are expectations on them to make sure that their vision, their capacity of compromise in discussion for reaching a peaceful solution in Syria is now, and they need to produce that.” The Geneva conference, he said, “cannot fail.”

Meanwhile, Sen. Bob Corker (R-Tenn.), chairman of the Senate Foreign Relations Committee, described the “tremendous frustration” of the Syrian opposition, expressed to him in a Wednesday telephone call from Riyad Hijab, the former Syrian prime minister who is head of the HNC.

“From their perspective,” Corker said in an interview, by not insisting that some confidence-building measures be taken, “the administration is trying very much to align itself with Russia and other entities to [the opposition’s] disadvantage. . . . A process that prematurely forces the Syrian opposition to the table without taking into consideration their concerns — such as humanitarian access and the release of women and children wrongly imprisoned in Syria — will not produce the desired outcome.”

“They were not railing against Secretary Kerry, they were not railing against anyone” in the call, Corker said. “They were just making their point. If they go to the talks under the situation as it exists, it is destined for failure. . . . I do not think they’re being unreasonable.”

Invitations to the talks have been issued by de Mistura to the HNC and to the government, each of which is to send 15 representatives. The two sides will not meet face-to-face, he said this week, but in “proximity,” in separate rooms with U.N. representatives passing back and forth between them.

In an effort to resolve disputes over who would be on the opposition team — with Russia and opposition backers demanding that other Syrian actors participate — de Mistura issued separate invitations to other groups to attend as advisers, who would sit apart from the two delegation rooms.

Sly reported from Geneva.