PRETORIA (Reuters) - U.S. Secretary of State Hillary Clinton said on Tuesday that the crisis in Syria must not be allowed to descend into sectarian warfare and she warned against "proxies or terrorist fighters" being sent in to join the conflict.

"We have to send very clear expectations about avoiding sectarian warfare. Those who are attempting to exploit the situation by sending in proxies or terrorist fighters must realize that will not be tolerated," Clinton said at a news conference in the South African capital Pretoria.

The U.S. Secretary of State, who is on a tour of Africa, 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.

Clinton said the international community must step up work on planning for a post-Assad Syria.

"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.

(Reporting by Andrew Quinn; Editing by Pascal Fletcher)