Secretary of State Hillary Rodham Clinton looks toward South Africa's Foreign Minister Maite Nkoana-Mashabane, during a summit in Pretoria, South Africa on Aug. 7, 2012. (Jacquelyn Martin/AP)

Secretary of State Hillary Rodham Clinton said Tuesday that the world must plan urgently to prevent sectarian warfare or imported terrorism in Syria after President Bashar al-Assad’s regime falls, citing the pace of government defections and the fact that Syrian rebel gains have opened up a safe corridor to Turkey.

Clinton would not predict how long Assad can hold on in the face of what she called an increasingly better-organized and better-equipped rebel force, but 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 during a diplomatic visit to South Africa.

“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,” she added.

Clinton is to hold emergency talks on Syria in Turkey this weekend, with discussions focusing largely on the crucial period immediately after Assad’s eventual fall, U.S. officials said.

The talks are also likely to include the new possibilities for getting aid or other assistance into Syria along a suddenly clear route from Turkey. Rebels hold much of the territory around Aleppo, as Clinton noted Tuesday.

“The opposition is becoming increasingly organized and effective,” Clinton said. “It now reportedly holds territory from northern Aleppo to the Turkish border.”

The United States appears more willing to use that corridor to help the rebel fighters, but Clinton did not address the fighters’ long-standing request for U.S. military help. The Obama administration has supplied advanced communications equipment but has stopped short of supplying guns or other lethal weapons.

Apart from Israel, Turkey is the strongest U.S. ally directly affected by the unfolding events in Syria.

Clinton’s decision to add a day of talks with top Turkish officials to an unrelated Africa trip is a mark of the Turks’ worry about the potential for a prolonged civil war at its doorstep and about the refugees already streaming into Turkey. It is also a sign of the deepening involvement of the United States as some kind of end to the Assad era comes into view.

“The Syria regime is crumbling and losing its grip on power,” State Department spokesman Patrick Ventrell said Monday.

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.

She has held several preliminary meetings with Syrian opposition groups, but the Obama administration has been wary of getting too closely involved.

The United States remains hobbled by poor knowledge of the Syrian factions battling the Assad government and sometimes one another.

“We have to make sure that we send very clear expectations about avoiding sectarian warfare,” and reprisals, Clinton said.

She said that warning applies equally to the government and the opposition and that “state institutions” must not be dismantled. That was a reference to the Obama administration goal of preserving enough of Assad’s military and government structure to forestall a collapse like the one in Iraq following the fall of Saddam Hussein.

Clinton also warned against outsiders trying to exploit a power vacuum by sending proxies or “terrorist fighters.”

“That will not be tolerated,” she said. She did not elaborate, but her remark appeared to be aimed at Iran.

“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.

On Monday, Syrian Prime Minister Riad al-Hijab resigned his post and fled to Jordan. The defection followed that of a top Syrian intelligence official and a prominent Syrian army hero in the past week.

Clinton wants South African support for tougher action on Syria at the U.N. Security Council, where South Africa holds a seat until the end of this year and often balks at supporting U.S.-backed initiatives.

South Africa abstained from voting on the most recent Security Council resolution on Syria, which would have called for sanctions for failing to comply with then-envoy Kofi Annan’s Syria peace plan. The resolution failed on vetoes by Russia and China.

Clinton acknowledged the disagreement.

“We do not always see eye to eye,” Clinton said Tuesday before meeting with South African Foreign Minister Maite Nkoana-Mashabane. “Sometimes we will disagree, as friends do.”

The foreign minister said that South Africa’s position on sanctions or other intervention has not changed but that the world should “quicken its steps” to help Syria choose what to do next.