A North Korean-flagged tanker laden with oil from a rebel-held terminal in eastern Libya slipped the warships deployed to intercept it and escaped to sea on Tuesday, MPs said.

The Morning Glory, which docked in Al-Sidra on Saturday and is reported to have taken on at least 234,000 barrels of crude, is the first vessel to have loaded oil from a rebel-held terminal since the revolt against the Tripoli authorities erupted last July.

The central government had threatened armed action, even an air strike, to prevent the tanker getting away with its cargo of oil bought from the rebels' self-declared autonomous regional government without the authorisation of the state-owned Libyan National Oil Corporation.

But members of the General National Congress, Libya's highest political authority, said that bad weather prevented the navy's small vessels from following the huge ship out into the Mediterranean.

"The oil tanker took advantage of poor weather conditions to head for the open sea. The ships that were surrounding it were not in a position to follow," one GNC member told AFP.

Abdelkader Houili, who sits on the GNC's energy committee, told Al-Nabaa television that the navy's warships, which mainly consist of fast patrol boats, had been forced to sail close to the coast because of the weather.

"The tanker then took advantage of the gap to head for the open sea," he said.

The Morning Glory's escape is a new humilation for the Tripoli authorities who have been battling to assert control over much of the country since the NATO-backed 2011 revolt that ended the 42-year dictatorship of Moamer Kadhafi.

In eastern Libya in particular, a myriad of former rebel militias, Islamist as well as regionalist, have carved out their own fiefdoms.

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

Former rebels calling for the restoration of the autonomy that Cyrenaica enjoyed for the first 12 years after Libyan independence in 1951, seized control of eastern export terminals last July.

- Washington 'deeply concerned' -

They have made several attempts to export crude through their own oil company but the Morning Glory is the first to have successfully taken on crude.

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

But Washington said on Sunday that it was "deeply concerned" over the loading of "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 Libyan National Oil Company and its joint venture partners."

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 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 within a week.

But analysts warned that any resort to force risked plunging Libya back into civil war and wreaking major damage to the country's key oil infrastructure.

The standoff between the Tripoli authorities and the eastern rebels has already slashed Libyan oil exports from 1.5 million barrels per day to just 250,000, dealing a massive blow to the North African country's key revenue earner.

The simmering tensions in Libya helped push up oil markets Tuesday with New York's main contract, West Texas Intermediate (WTI) for delivery in April, gaining 19 cents to $101.31 a barrel.

"Libya continues to support prices as well, with the threat to sink a North Korean tanker if it loads oil from a rebel held port being taken seriously in the markets," said analyst Joe Conlan at energy consultancy Inenco.