The collapse of the U.N. initiative on Syria, rebel gains that opened a corridor from Turkey to Aleppo, and a rash of high-level defections mark a turning point in the Syrian crisis and in the Obama administration’s plans for influencing the outcome.

While some U.S. officials, particularly inside the State Department, are pushing for more direct assistance to the Syrian opposition, for now, administration policy remains focused on nonlethal aid and planning for the day after the fall of Syrian President Bashar al-Assad.

In a hastily arranged trip to Turkey this weekend, Secretary of State Hillary Rodham Clinton plans to meet with Syrian opposition figures beyond the exile leaders who have been the public political face of the insurgency, a senior administration official said.

“Now that we’ve turned away from the United Nations, and there’s a lot going on” inside Syria, Clinton wants to talk to “activists on the ground,” the official said.

In remarks Tuesday during a visit to South Africa, Clinton did not address long-standing demands by rebel fighters for U.S. military help. Instead, she said that “the intensity of the fighting in Aleppo, the defections, really point out how imperative it is that we come together and work toward a good transition plan.”

On Monday, Syrian Prime Minister Riyad Hijab resigned his post and fled to Jordan, part of a wave of defections that have depleted Assad’s ranks.

Despite the defections and rebel gains, a second U.S. official involved in the discussions said the administration remains unsure of the identities and motives of many opposition figures inside Syria. The official also said there are continuing changes in intelligence on which groups are in control of particular villages and districts inside Syria.

The official and others interviewed for this article spoke on the condition of anonymity because of the sensitive nature of the information.

“It’s not as though there’s a government in waiting or some secret commander somewhere,” the administration official in Washington said.

While Clinton does not plan to meet with representatives of the Free Syrian Army, the primary rebel military organization, her stop in Turkey is designed to give her more visibility into opposition figures directly involved inside Syria. She will also talk with Turkish officials who are more directly involved with the opposition movement.

Clinton’s decision to stop in Turkey on Saturday at the end of an unrelated Africa trip also reflects Turkey’s concern about the potential for a long civil war at its doorstep and the problems created by the tens of thousands of Syrian refugees streaming across its border.

The Turks “don’t know what they want,” said Henri Barkey of Lehigh University. “We don’t have a clear policy. Why do we expect them to have a clear policy?” The next step is “military activity,” he said, “and they don’t want to do it. Even though they are very much against Assad, there’s no support for that in Turkey.”

Discussions are ongoing among the administration, its allies and the opposition about whether it is feasible, or even desirable, to try to protect rebel gains around the Syrian city of Aleppo and the newly opened corridor between it and the Turkish border.

“There are implications attached to it,” said Sen. John F. Kerry (D-Mass.), whose Foreign Relations Committee last week received the latest in a series of classified administration briefings on the subject.

“Once you’ve made that kind of move,” Kerry said, “you have to have a full determination of how you back it up” if Assad decides to contest it. Although the road from Aleppo is under rebel control, it remains under attack at a distance from government artillery and air power.

“I don’t think it’s a ‘no way, Jose’ situation,” Kerry said about possible intervention. “But there are limits, and the constraints are very real. That is not to say there isn’t a fair amount of energy being put into how do you change the calculus for Assad on the ground.”

The administration is considering several options, including increasing intelligence-sharing with the Free Syrian Army and providing additional communications equipment. Other options are using air power to protect the corridor to Aleppo and increasing the weapons being provided to the rebels via Saudi Arabia and Qatar, among others.

Citing the pace of defections and rebel momentum, Clinton addressed the urgent need for plans to prevent sectarian warfare or imported terrorism in Syria after Assad’s inevitable fall, and she suggested that the United States is accelerating planning for what she called “the day after.”

“I do think we can begin talking about and planning for what happens next,” she said in an appearance with South African Foreign Minister Maite Nkoana-Mashabane.

U.S. officials do not hold a uniform view of Assad’s longevity, but Clinton is concerned that a quick fall could open a dangerous vacuum.

Clinton also warned against outsiders trying to exploit a power vacuum by sending proxies or “terrorist fighters.” In an apparent warning to Iran, she said that “that will not be tolerated.”

In her talks with the opposition, the senior administration official said, Clinton will emphasize the need to keep Syria’s security services and police intact to avoid the kind of chaos that resulted when those institutions were disbanded following the U.S. invasion of Iraq. Syrians “will want water, food, humanitarian [relief] and medical care,” the official said, and “we know that no matter who comes [into power], we want to be thinking about what we need to put our embassy staff back into Damascus.”

“We have to think about what we can do to support a Syrian-led democratic transition that protects the rights of all Syrians,” Clinton said, adding that she will discuss that priority in Istanbul on Saturday.

Gearan reported from Pretoria, South Africa.