Libyan authorities said they intercepted a North Korean-flagged tanker Monday which had loaded crude from a rebel-held eastern port, as the conflict over the country's key petroleum revenues escalated.

Former rebels calling for autonomy for the eastern Cyrenaica region have been blockading the port of Al-Sidra and other key eastern export terminals since July.

But the North Korean-flagged Morning Glory, which docked in Al-Sidra on Saturday, was the first ship to take on crude from a rebel-held port since the standoff with the central government began.

Officials threatened armed action if necessary to stop it putting to sea and announced that a military task force was being formed to bring the rebel ports back under government control.

"Naval and revolutionary forces are in control of the tanker Morning Glory and are escorting it to a state-controlled port," the country's top legislative and executive body, the General National Congress, said on its website.

But rebel spokesman Ali al-Hassi denied the statement, saying the ship was still in rebel hands at the port, and a spokesman for the Libyan navy, Ayub Omar al-Kassem, said the boat was surrounded by government forces but had not left the port's boundaries.

Oil Minister Omar al-Shakmak had said earlier that the ship had interrupted loading late Sunday and put back to sea.

The 350,000 barrel-capacity vessel had only taken on 234,000 barrels of crude, according to a member of a government crisis team.

Warships had deployed to block the Morning Glory after Culture Minister Amin al-Habib warned on Sunday the tanker would be "turned into a pile of metal" if it tried to leave port.

The defence ministry had also deployed the air force, the official LANA news agency said.

Washington said Sunday it was "deeply concerned" over the loading of the "illicitly obtained" oil.

"This action is counter to law and amounts to theft from the Libyan people," State Department spokeswoman Jen Psaki said.

"The oil belongs to the (state-run) Libyan National Oil Company and its joint venture partners."

- Breaking the blockades -

Libya's weak central government has struggled to rein in the former rebels who toppled longtime dictator Moamer Kadhafi in a 2011 uprising.

Ex-rebels at Al-Sidra spent the weekend loading oil onto the tanker, ignoring Prime Minister Ali Zeidan's threats to bomb the ship.

It was not the first time that a foreign tanker had tried to dock at a rebel-held terminal but it was the first time that one had taken on oil.

Former rebels have been demanding a return to the autonomy that Cyrenaica enjoyed under the federal constitution that Libya adopted for the first 12 years after independence in 1951.

"We are not demanding the breakup of the country," the head of the federalists' self-declared autonomous government, Abdrabbuh al-Barassi, said Saturday.

"The oil revenues would be split between the three autonomous regions," he said, promising that all sales would be carried out with complete transparency.

The GNC said a task force composed of both regular troops and ex-rebel militia was being formed to bring the rebel ports back under central government control.

The crisis erupted in July, when ex-rebel security guards at key terminals shut them down, accusing the authorities of corruption and demanding a more equitable distribution of oil revenues.

The demands of the protesters swiftly escalated with a call for the eastern Cyrenaica region to be granted autonomy and the right to enjoy the revenues from its own oil exports.

Oil is a key source of revenue for Libya and following the blockade of terminals production plunged from 1.5 million barrels per day to just 250,000.

The central government exerts little control over Libya's east, the cradle of the 2011 uprising, where a myriad of former rebel militias, Islamist as well as federalist, have carved out their own fiefdoms.

There have been almost daily attacks on security and other government personnel in the region's main towns.