Libya has threatened to bomb a North Korean-flagged tanker at an oil terminal held by militants in the restive east, saying it was being loaded illegally.
Separatists blockading Al-Sidra facility attempted to load crude aboard the Morning Glory in the latest challenge to central government control.
"The attorney-general has given the order for the ship to be stopped," Prime Minister Ali Zeidan told a news conference on Saturday.
"All parties must respect Libyan sovereignty. If the ship does not comply, it will be bombed," he said.
Zeidan said the authorities had told the vessel's captain to leave Libyan territorial waters, but he said armed gunmen on board were preventing him from leaving.
The self-proclaimed government of Cyrenaica in the east, the political wing of the separatists, said oil exports from Al-Sidra had now begun.
"We announce to Libyans and to the whole world that we have begun exporting oil," said Rabbo al-Barassi, who heads the Cyrenaica executive bureau formed in August by federalist activists.
"We are not defying the government or the Congress (parliament). But we are insisting on our rights," he said.
Protesters at the eastern oil ports are demanding a restoration of the autonomy the eastern region was granted in the first decade after Libya's independence in 1951.
They have also accused the authorities of corruption and are demanding a more equitable distribution of oil revenues.
Deputy Defence Minister Khaled al-Sherif said a "crisis committee" of government officials and politicians had issued an ultimatum for the oil tanker to depart.
"If the ship doesn't comply, it will be bombed by the air force or intercepted at sea by the navy," Sherif warned.
An MP and committee member said the 2pm (2300 AEDT) deadline expired without any action being taken, however.
Washington's envoy to Tripoli warned on Twitter that any cargo deemed to have been loaded illegally would face international sanction.
