Libya threatened on Saturday to bomb a North Korean-flagged tanker at an oil terminal in the restive east if it does not leave port, saying it was loading illegally.
Separatist militants blockading Al-Sidra facility in eastern Libya attempted to load crude aboard the ship the Morning Glory in the latest challenge to central government control.
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 told AFP a "crisis committee" of government officials and lawmakers had issued an ultimatum for the oil tanker to leave Libyan territorial waters.
"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 2:00 pm (1200 GMT) 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.
"Companies that engage in illicit trade with separatist groups in Libya risk liability in multiple jurisdictions," US Ambassador Deborah Jones tweeted.
"The only entities authorised to sell Libya's oil are the Libyan National Oil Corp., its subsidiaries JV partners," she added.
Earlier, a member of the energy committee in Libya's interim parliament, the General National Congress, said an "oil tanker, bearing the name Morning Glory, anchored on Saturday at 4:00 am (0200 GMT) at the port of Al-Sidra".
Militants seeking autonomy for Libya's east were trying to load a shipment of petroleum "outside the framework of the state", added the GNC member, speaking on condition of anonymity.
Acting Oil Minister Omar Shakmak denounced the move as an "act of piracy".
"This is a violation of national sovereignty. It is up to the defence ministry to deal with this ship," he told AFP, without elaborating.
The latest crisis erupted in July, when security guards at key oil terminals shut them down, accusing the authorities of corruption and demanding a more equitable distribution of oil revenues.
The situation has become more complicated as self-rule activists in the east have insisted on the right to export.
Saturday's incident was the latest in a stand-off between the government and militants over exports, which are the principal source of revenue for the North African country.
Oil installation guards launched the blockade of key petrol sites last July, initially over price hikes but later also to call for autonomy for Cyrenaica, cradle of the 2011 revolt that toppled Muammar Gaddafi.
In January, the navy prevented two tankers from docking in Al-Sidra to take on crude. The government also threatened to bomb any ships attempting to dock without permission from the National Oil Corporation.
Following the blockade, production plunged to around 250,000 barrels per day from 1.5 million bpd, and the economy ministry estimated the treasury has lost more than $9 billion (6.5 billion euros) in revenue.
Production has since recovered to 546,000 bpd, but the crisis continues to take hit state revenues.
With independence after World War II, Libya was divided into three regions -- Cyrenaica in the east, Fezzan in the southwest and Tripolitania in the north.
Those regions were done away before Gaddafi seized power in 1969, but they were not forgotten.