KHARTOUM South Sudan accused Sudanese President Omar Hassan Al-Bashir of arming militias to overthrow the south's government before the secession of the oil-producing region in July, and suspended talks with Khartoum.
The accusation, dismissed by the north, came hours after militias launched an attack on Malakal, capital of the south's oil-producing Upper Nile state.
Senior southern official Pagan Amum said the south would suspend talks with Bashir's ruling National Congress Party (NCP) about plans for the secession and would look into alternative routes for sending its oil to market, away from the north.
Southerners overwhelmingly voted to declare independence from the north in a January referendum promised in a 2005 peace deal that ended decades of north/south civil war. The south is due to split away on July 9.
"We in SPLM (the south's ruling Sudan People's Liberation Movement) have details of a plan by the NCP to overthrow the government of south Sudan before July," Amum, SPLM secretary general, told journalists.
"The NCP has been creating, training, supplying and arming militia groups in southern Sudan with the aim to destabilise and overthrow the government," he added. "This plan is being overseen by the President of the Republic ... himself."
Amum said the SPLM had complained to the United Nations Security Council about a series of militia raids in its territory which it said were being backed by Khartoum.
"For all these reasons, the SPLM is suspending discussions and negotiations with the NCP," said Amum.
Amum added southern President Salva Kiir had asked him to look into finding alternative routes for the south's oil -- the lifeblood of both north and south economies.
"(Kiir) has directed us in the negotiating team to look into a possibility of stopping the export of oil of south Sudan through the north in July and see possibilities of alternative routes of transport other than northern Sudan," said Amum.
It was unclear how the south, almost entirely dependent on oil revenues, would manage to find alternative routes. The only pipeline currently runs through the north and it would take years to build another line via Kenya or Uganda.
Any shutdown would have a serious impact on the north's economy which, up to now, has been receiving half of the revenues from southern oil and was hoping to negotiate continued payments after secession.
Senior NCP official Rabie Abdelati told Reuters the accusation of a northern plot was "ridiculous."
If the government of the south wants to export oil through any other means, it is up to them. We don't want to take something that is not ours," Abdelati told Reuters.
The south's army said 23 militia fighters died after they raided Malakal, killing an unknown number of civilians.
Renegade militia leader George Athor told Reuters one of his deputies had launched the assault to seize weapons and to strike back after a series of army offensives against his men.
Athor is a former army officer who rebelled last year saying he had been cheated out of the governorship of neighboring Jonglei state in April elections. The south has accused the north of backing Athor, an accusation he dismisses.
Upper Nile includes oil concessions run by Petrodar, a consortium led by CNPC of China and including Malaysia's Petronas and Sudan's own Sudapet.
(Additional reporting by Jeremy Clarke in Juba)