Sudanese President Omar al-Bashir has threatened to shut pipelines carrying South Sudan's oil if a deal on oil is not reached before it secedes in July.
He said either the south could continue to hand over half of its oil revenue to the north, or it could pay for using the north's oil infrastructure.
Mr Bashir warned that if neither was accepted, he would block the pipeline.
Three-quarters of Sudan's oil is in the south, but most pipelines, refineries and the main port are in the north.
Southerners voted for independence in a referendum in January.
President Bashir made his threat in a speech at a rally in Port Sudan, the main oil export terminal.
"I give the south three alternatives for the oil," he said.
"The north is to continue getting its share, or the north gets fees for every barrel that the south sends to Port Sudan," he added.
"If they don't accept either of these, we're going to block the pipeline."
The BBC's James Copnall in Khartoum says oil accounts for about 98% of the south's income, so any reduction in the oil flow would be disastrous.
The government of South Sudan has floated the idea of building a new pipeline through Kenya or Uganda, but this would take several years, our correspondent adds.
Talks are continuing between northern and southern Sudan about oil and other vital pre-independence issues, including citizenship and the disputed border region of Abyei.
Last month, the northern army seized control of the disputed region of Abyei, but a deal was reached on Monday which will see it withdraw and be replaced by Ethiopian peacekeepers.
A new report published by the House of Lords, the UK's upper chamber of parliament, highlights the many issues that will bedevil future Sudanese relations, including oil.