* Top U.S. diplomat rejects "proxies, terrorists" coming in
* Now is time to plan for "day after" Assad - Clinton
* Clinton seeks to bring S.Africa on board with Syria plan
By Andrew Quinn
PRETORIA, Aug 7 (Reuters) - Syria must not be allowed to descend into a sectarian war, U.S. Secretary of State Hillary Clinton said on Tuesday, and she warned against "proxies or terrorist fighters" being sent in to join the conflict.
"We have to set very clear expectations about avoiding sectarian warfare," Clinton said, answering a question on Syria during a news conference in the South African capital Pretoria, her latest stop in a tour of Africa.
"Those who are attempting to exploit the situation by sending in proxies or terrorist fighters must realize that will not be tolerated, first and foremost by the Syrian people," she added.
The U.S. secretary of state made the comments as Syrian government forces loyal to President Bashar al-Assad encircled rebels in the country's biggest city of Aleppo..
She did not elaborate on her reference to "proxies or terrorist fighters" or name any particular country or group.
But the escalating war in Syria has increasingly divided the region along its sectarian faultline, pitting the mainly Sunni rebels, who are backed by regional Sunni-led powers Turkey and the Gulf Arab states, against Assad's government that is backed by Shi'ite Iran.
Iran assured the Syrian president on Tuesday that it viewed his country as a vital partner.
With action on Syria at the United Nations Security Council effectively blocked by vetoes from Russia and China, the United States and its allies have pledged to increase political support for the Syrian opposition.
Clinton said the United States had taken note of the defection on Monday of Assad's Prime Minister Riyad Hijab, who fled the country and denounced the "terrorist regime" in Damascus.
The defection of Hijab, who like most of the opposition hails from the Sunni Muslim majority, was a further sign of the isolation of Assad's government around an inner core of powerful members of his minority Alawite sect.
"That's the latest in a line of such defections and the opposition is becoming increasingly coordinated and effective. It now reportedly holds territory from northern Aleppo to the Turkish border," Clinton said, adding the rebels had also seized weapons from the government, including tanks.
Clinton said the opposition gains, combined with the fracturing of some of Assad's support, made it imperative for the world to step up work on planning for a post-Assad Syria.
"We can begin talking about and planning for what happens next, the day after the regime does fall. I'm not going to put a timeline on it, I can't possibly predict it, but I know it's going to happen," Clinton said.
"We must figure out ways to hasten the day when the bloodshed ends and the political transition begins ... We have to make very sure that state institutions stay intact," she said.
Clinton said she intended to discuss the challenges facing Syria when she flies to Turkey for talks on Saturday. Turkey has been a vocal critic of Assad's internal crackdown, but will also host Iran's foreign minister for separate talks. .
The U.S. secretary of state also discussed the Syria issue with South African Foreign Minister Maite Nkoana-Mashabane, whose country has expressed doubts about the U.S.-led drive to impose tough international measures on Assad.
South Africa joined Pakistan in abstaining during the latest U.N. Security Council vote on possible sanctions on Damascus, saying the draft resolution was unbalanced because it only targeted the government and not the rebels. This echoed similar Russian and Chinese complaints.
Nkoana-Mashabane told the news conference in Pretoria that South Africa could only support a "made-in-Syria" solution to the crisis that did not involve foreign intervention.
"South Africa's position is and has always been that no amount of bloodshed will ever take the place of a political solution," she said.