Armed federalists forced two oil terminals in east Libya to halt production, sources said, days ahead of the first elections since a popular uprising toppled the regime of Moamer Kadhafi.
"The harbour is closed... The pumping and loading of oil has been stopped... The group that came were federalists," said Tumi Shakari, a supervisor at a major oil terminal in the port town of Ras Lanuf.
The action follows threats from a pro-federalism movement to boycott and sabotage the July 7 poll, the first since last year's uprising.
"A group of 15 people came around 9:30 pm (1930 GMT) and in a very peaceful and amicable manner asked us to shut down operations," the supervisor added.
"This group has certain demands that they want to see fulfilled and they have asked us to stop our work for 48 hours," he continued, adding that workers had to decided to comply to avoid an escalation.
"We are waiting for orders from the National Oil Corporation," he said.
A witness said the armed men had arrived in national army vehicles.
One of the central demands of the federalist movement is an equal allocation of seats in the 200-member national assembly which will be elected on Saturday.
The interim authorities, on the basis of demographic considerations, gave 100 seats to the west, 60 to the east and 40 to the south.
Ibrahim al-Jadhran, a protest leader, said demonstrators had also shut the port of Al-Sidra, 35 kilometres (20 miles) west of Ras Lanuf, and were heading eastbound to the oil terminal of Brega.
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Tareq al-Tahi, senior superintendent at Al-Sidra, confirmed that the terminal had been shut.
"We have been forced to stop production," he told an AFP reporter on site.
"At 7:30 pm (1730 GMT) a group of people came to us in armoured vehicles loaded with anti-aircraft guns and asked us to stop producing oil and loading cargo," Tahi added.
"The situation is the same in Ras Lanuf, Brega, and Haruj (near Ras Lanuf)."
Production of several oil companies are routed from the port of Ras Lanuf, including Libya's AGOCO (55,000 bpd), German company Wintershall (70,000 bpd) and Al-Haruj (80,000 bpd), according to industry experts.
The oil terminal complex of Ras Lanuf lies 360 kilometres (225 miles) west of the eastern city of Benghazi, cradle of the revolt that ousted Kadhafi, who was captured and killed last October.
Libya's oil production is up to 1.55 million barrel per day, nearing pre-war levels, according to government officials.
A former rebel manning a checkpoint on the coastal highway said: "We closed the terminal because we want our demands to be fulfilled. This is an issue of seat allocations."
Earlier on Thursday, arsonists set fire to a depot holding electoral material in the eastern city of Ajdabiya, which lies between Benghazi and Brega on the Mediterranean.
And armed federalists backers on July 1 ransacked offices of the electoral commission in Benzghazi raising concerns that the vote could be disrupted by such groups.