Libya's Prime Minister Ali Zeidan warned on Wednesday oil tankers trying to reach ports seized by armed protesters must stay away or they could be sunk by the navy.

He made the statement after the navy fired warning shots to ward off a tanker that the National Oil Corp (NOC) said tried to load crude at a terminal held by protesters, who say they opened ports under their control for business.

"Any country or company or gang trying to send tankers to take oil from the seized ports without coordinating with the NOC, we will deal with them, even if we are forced to destroy or sink them," Zeidan said. "We warn all countries there will be no leniency."

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

The group in eastern Libya said it would invite foreign companies to buy oil from seized ports and protect arriving tankers, challenging Tripoli.

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.

"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. - Reuters