A heavily armed autonomy group in eastern Libya said it would invite foreign companies to buy oil from seized ports and protect arriving tankers, challenging Tripoli which has promised to use force to stop them.

The announcement came just hours after Libya's defence ministry said it would destroy any tankers loading oil from eastern ports in the Cyrenaica region which are under control of the armed protesters.

The escalation adds to chaos as the weak Tripoli government struggles to rein in armed groups that helped oust Muammar Gaddafi in 2011 but kept their guns to demand power and a bigger share of the country's oil wealth.

The conflict is hurting oil revenues, which fund the Opec nation's government and the import of wheat and other crucial food. The government has warned it will be unable to pay public salaries if the standoff continues, risking more turmoil.

On Monday, Libya's navy said it had shot at a tanker trying to load oil at the eastern Es-Sider port which was seized with two other terminals by the autonomy group in August. The three harbours accounted previously for 600,000 barrels a day.

But the group, led by tribal leader and 2011 civil war hero Ibrahim Jathran, shrugged off Tripoli's warning by inviting foreign companies to buy eastern oil.

"We welcome global oil companies ... The oil security guards will guarantee the safety of tankers," said Abd-Rabbo al-Barassi, prime minister of Jathran's self-declared eastern government.

Workers at the seized ports had returned to work, he said, without giving details on when they had left. He added that a newly founded oil company called Libya Oil and Gas Corp would be dealing with potential buyers. A new army and coast guard, made up of Jathran's battle-hardened fighters, would secure the ports.

Barassi said his group had nothing to do with the tanker shot at by a Libyan navy vessel on Sunday on its way to Es-Sider. Tripoli has said the tanker had been bound to load oil at the seized port, but Barassi called this statement a "lie."

The confrontation has raised worries that Libya, also struggling with Islamist militias and armed tribesmen, might break apart as the eastern Cyrenaica and southern Fezzan regions demand political autonomy.

But Barassi, speaking on a pro-autonomy television station late on Tuesday, said his group had no secession plans.

"We confirm we want to preserve Libya's territorial unity," he said, adding that it invited Tripoli to send a delegation to help oversee the proper sale of oil sales. "We assure all Libyans that the sale of oil will be according to the law." - Reuters