* Marsa Al Hariga port reopens
* NOC declares force majeure at four ports
* PFG defuses protest, fires rogue Es Sider leader
* Tanker off Es Sider under observation
* FACTBOX on status of Libyan oil installations (Updates with force majeure)
By Feras Bosalum and Julia Payne
TRIPOLI/LONDON, Aug 19 Libya was forced to suspend contractual obligations with a force majeure on some oil exports on Monday, acknowledging weeks of disruption which has cut shipments to their lowest since the civil war of 2011.
Yet the reopening of the Marsa al Hariga oil port - if it holds - may also show the first fracturing of strikes.
The intervention from a new leader of the oil security body that protects installations, the removal of a protest leader who had threatened to sell oil himself and some port workers rejecting further disruption, could be the first faint signs of a turn in the tide.
Oil traders remained sceptical the chaos will end soon.
About half of Libya's over 1.2 million barrel per day export capacity remains shut down, industry sources said.
The crisis hit a peak at the end of last week when the government threatened military action should striking security guards at Es Sider, Libya's largest terminal, sell oil independently.
Marsa al Hariga resumed full operations and is ready to export following strikes, an oil ministry spokesman said on Monday.
The 110,000-barrel-per-day (bpd) port has been mostly shut for the last three weeks due to worker protests, including employees of operator Arabian Gulf Oil Co, with only some fuel shipments for domestic use leaving the terminal.
It was not immediately possible to reach Arabian Gulf Oil Co who participated in closing the oilfields, which feed the port and Libya's largest refinery.
"We need to see a vessel berth and load," one trader said, who has a tanker waiting nearby.
Force majeure was declared on crude oil and oil product exports from their Zueitina, Marsa al Brega, Ras Lanuf and Es Sider terminals, a Libya National Oil Corp document showed.
DIVISIONS AMONG PORT WORKERS
But the striking guards were under mounting pressure from colleagues at other local ports, several Libyan sources said, and there were no signs they have gained wider support to sell oil independently.
A new chairman of the Petroleum Facilities Guard (PFG), many of whose members are striking, has taken the lead in negotiations.
Edris Abokhamada became head of the PFG at the start of August, replacing Rasheed Al Sabri.
"(Edris) succeeded to defuse the strike at Tobruk and Marsa al Hariga and now he's working on Zueitina," one senior Libyan oil official told Reuters.
The loyalty of the PFG heads and disunity among oil workers in the east is leaving the leader of the Es Sider and Ras Lanuf protest somewhat isolated, sources said.
"We received a letter from employees at the Brega port saying they are against the port closures and will work under the government," a PFG spokesman said.
The government said last week that Ibrahim al-Jathran, head of the middle region for the PFG, was seeking to sell oil for the benefit of his group of striking oil workers.
"Ibrahim has been removed from his position by the chief of staff," the spokesman added, and said he expected the other ports to reopen soon.
"The idea of some workers coming out against the protests is interesting, as it shows there isn't a completely unified position supporting the closure, but that could easily lead to more disputes and divisions rather than speed up the reopening process," said Richard Mallinson, chief policy analyst at Energy Aspects.
The PFG operates under the Defence Ministry to protect oil installations, but only about 2,000 of its 15,000 members have had training from the military.
Es Sider has around eight crude oil tankers waiting to load at anchor.
Trading sources and employees at Es Sider said there was one tanker, A Whale, waiting outside that was unaccounted for and market sources did not know who had chartered it. Oil sources in Libya speculated the ship may be linked to attempts to sell oil independently.
"We are calling the tanker, but we are not able to reach the captain," one of the port workers said.
Libyan authorities increased their profile at Es Sider. A Libyan Coast Guard vessel, the Toukra Tess, entered the port on Saturday, maritime analytics firm WindWard said. (Additional reporting by Ron Bousso; editing by William Hardy)