South Sudan says it is suspending talks with Khartoum, accusing Sudanese President Omar al-Bashir of plotting to overthrow the southern government.
A senior official, Pagan Amun, said the plot was being orchestrated by the north's military intelligence. The north has denied the accusation.
The statement followed fighting between southern forces and a rebel militia in the southern oil town of Malakal.
South Sudan is to declare independence in July following January's referendum.
Mr Amum said the south's ruling Sudan People's Liberation Movement (SPLM) had details of a plan by Mr Bashir's National Congress Party (NCP).
"The NCP has been creating, training, supplying and arming militia groups in southern Sudan with the aim to destabilise and overthrow the government before July," he claimed.
"This plan is being overseen by the president of the republic Omar Hassan al-Bashir himself."
Earlier on Saturday, supporters of a renegade general leading a rebel militia in the south, Gen George Athor, attacked Malakal in the oil-producing state of Upper Nile.
It is the first major town to be attacked by the rebels. Eleven people are reported to have been killed.
Mr Amum also said the southern government was looking into ways of diverting oil routes that usually go through the north.
But a ruling party official in the north denied the accusation of a plot as "ridiculous".
Rabie Abdelati, a senior member of the National Congress Party added: "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," he told Reuters news agency.
Gen Athor went into rebellion after losing last April's election to be governor of Jonglei state, which he contested as an independent candidate.
His rebellion appears to be spreading, and represents a growing concern for the Southern authorities as they prepare for independence, our correspondent says.