(Adds force majeure extended at three ports)
TRIPOLI, Jan 4 (Reuters) - Libya warned customers against buying any crude offered by a pro-autonomy group which seized three oil ports in the east of the OPEC producer more than five months ago, the state oil firm said on Saturday.
The heavily-armed group calling for autonomy and a greater share of oil sales is occupying the eastern Ras Lanuf, Es-Sider and Zuweitina ports, which previously accounted for 600,000 barrels a day of oil exports.
Libya's output has fallen to around 250,000 bpd from 1.4 million bpd in July due to strikes at oil facilities across the North African country.
The eastern group, led by a militia leader who helped topple Muammar Gaddafi in 2011, has said it might sell crude on its own unless the Tripoli government meets its demands to share power and oil revenues.
"The National Oil Corp (NOC) warns everybody not to send ships to load or discharge at these (three) ports," the state oil firm said in a statement.
NOC would take legal action against the signing of petroleum purchase agreements with another party, it said.
The status of force majeure, a legal term to cover the suspension of contractual obligations, had been extended at all three ports, NOC added.
Western powers fear the North African country will slide into instability as the government struggles to rein in militias that helped topple Gaddafi but kept their arms.
Prime Minister Ali Zeidan has said the government will act against the oil strikes but Libya's nascent army, still in training, is too weak to tackle armed protesters, analysts say. (Reporting by Ulf Laessing; Editing by Rosalind Russell)