(Refiles to additional subscribers)
* Zeidan warns army has orders to break up port blockade
* El Sharara oilfield production also halved by weather
* Analysts see military operation difficult at ports
By Ghaith Shennib
TRIPOLI, Feb 3 (Reuters) - Libyan Prime Minister Ali Zeidan stepped up the pressure on protesters blocking eastern ports on Monday, telling them he had weeks ago ordered troops to prepare to move there to end their blockade.
Zeidan has repeatedly warned he may use force to free up three key ports where protesters demanding more autonomy from Tripoli have cut off around 600,000 barrels per day of oil exports since summer.
"Weeks ago we ordered the minister of defence to give his instructions to the chief of staff to move toward the occupied ports in the east," Zeidan said at a news conference. "Now the matter is in the hands of the army command."
He gave no further details.
An army spokesman did not immediately respond to calls seeking details of any troop movements.
Armed protesters who defected from the state-run Petroleum Facilities Guard (PFG) in August seized Ras Lanuf, Es Sider and Zuetina ports, led by Ibrahim al-Jathran, a former rebel who fought against Muammar Gaddafi in Libya's 2011 uprising.
The oil port standoff is one of several disputes involving rival militias and former rebels who fought in the civil war against Gaddafi but have since refused to disarm and now use their military muscle to make demands on the state.
Protests at oilfields and installations have battered the OPEC country's economy, cutting off the key source of state revenues just as the fragile government is struggling to gain momentum in the country's transition to democracy.
Zeidan managed to negotiate the end of another protest in the west late last year to restart production at the El Sharara oilfield with 340,000 barrels per day coming back online.
Production at El Sharara was taken down to 175,000 bpd on Monday because of closure of western ports due to bad weather, a spokesman for the state-run National Oil Corporation said.
Negotiations have gone nowhere with the eastern federalists who have set up their own self-styled Cyrenaica government. But local eastern tribal leaders and officials say support is waning for Jathran within the federalist movement.
An attempt to load a tanker at Es Sider port ended abruptly when the navy opened fire, making clear how difficult it would be for Jathran to sell oil independently of Tripoli.
But with Libya's nascent army still in training, most analysts say it will also be difficult for Zeidan to send troops to free up the ports, where Jathran has dug in with his own militia.
Two and a half years after the fall of Gaddafi, the brigades of former fighters and militias who still control parts of the country are one of the greatest challenges to Libya's attempts to forge its new democracy and a stable government. (Writing by Patrick Markey; Editing by William Hardy and Peter Graff)