* Tension mounts as unity, separation backers clash
* Referendum timetable tight but still "doable"-envoy
(Recasts with request for buffer zone)
By Louis Charbonneau
KHARTOUM, Oct 9 (Reuters) - South Sudan&${esc.hash}39;s president has asked U.N. Security Council envoys to send peacekeepers and set up a buffer zone along the country&${esc.hash}39;s north-south border ahead of a southern vote on independence, diplomats said on Saturday.
Sudan&${esc.hash}39;s oil-producing south is three months away from the scheduled start of a politically sensitive referendum on whether to secede or stay part of Sudan, a vote promised in a 2005 peace deal that ended decades of civil war with the north.
Sudan&${esc.hash}39;s Muslim north and its south, where most follow Christianity and traditional beliefs, have still not agreed on the position of their shared border and analysts fear conflict could re-erupt in contested zones, some of which contain oil.
In a sign of the tension over the vote, up to 40 pro-independence southerners clashed with riot police and northern unity campaigners in downtown Khartoum on Saturday.
The demonstrations were held to coincide with the visit to the capital of the envoys of U.N. Security Council states, who also visited the semi-autonomous south in recent days and met with its president, Salva Kiir.
"Salva Kiir asked for U.N peacekeepers to be deployed along the border between the north and the south," one diplomat told Reuters, on condition of anonymity, saying the request was made at a meeting in the southern capital Juba on Wednesday.
The diplomat said the request would be considered but the envoys made no promises to Kiir.
A second diplomat said Kiir had also proposed setting up a buffer zone along the ill-defined border. The diplomat did not give details of location of the proposed zone or its width.
A third member of the visiting Security Council delegation said a deployment of peacekeepers on the frontier was not something explicitly called for in the 2005 Comprehensive Peace Agreement and would therefore require some "adjustments".
The U.N. has 10,000 peacekeepers stationed in Sudan, not counting its joint mission with the African Union in the western province of Darfur. Most of the 10,000 are in the south and in three former civil war battle ground areas along the border.
RIOT POLICE
Riot police beat the group of southerners after they turned up at a 2,000-strong rally in support of Sudanese President Omar Hassan al-Bashir, timed to coincide with the Security Council envoys&${esc.hash}39; visit to the foreign ministry nearby.
Clad in orange caps, the southerners arrived at the rally shouting pro-independence slogans, witnesses said. Pro-unity supporters approached the southerners in downtown Khartoum, shouted at them to leave and pushed towards them.
Police beat the southerners with fists and batons and arrested some of them, witnesses said. Officials also hit three Westerners, two of them journalists, after ordering them to leave the scene.
Karti told the U.N. delegation Khartoum was committed to holding the vote on time but would not accept any southern "interference". Khartoum has accused the south of cracking down on unity supporters, a charge dismissed by the south.
Britain&${esc.hash}39;s ambassador to the U.N. Mark Lyall Grant later told reporters the timetable for the vote was "very tight" but "doable" if the parties stuck to their promises with backing from the international community.
Southerners, embittered by the conflict and perceived northern exploitation, are widely expected to choose secession. Khartoum wants to keep Africa&${esc.hash}39;s largest country united.
Troops from both sides have clashed since the accord, most recently in the contested Abyei oil region. The two sides have also accused each other of building up troops near their shared border as the referendum approaches.
(Additional reporting by Andrew Heavens and Khaled Abdel Aziz; Editing by Peter Graff)
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