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By Ayman al-Warfalli

BENGHAZI, Libya, May 3 (Reuters) - An oil company set up by Libya's eastern government is preventing a tanker from loading a cargo for its Tripoli rival, the National Oil Corporation (NOC), officials said on Tuesday.

The eastern company, also calling itself the National Oil Corporation, ordered workers at Marsa el-Hariga port in eastern Libya not to load the tanker, which had been waiting for two days, a port official said.

If the tanker, Seachance, cannot load or the port is closed, Libya's production could drop by 120,000 barrels per day, the official said.

An eastern NOC official said the order was in line with the east's failed attempt to export a shipment of 650,000 barrels of oil last week in defiance of the authorities in Tripoli, part of a power struggle between Libya's rival administrations.

Seachance had been initially due to load on April 26-28 and was part of the Tripoli NOC's loading programme, an NOC official in the capital said.

The eastern NOC was formed by one of two competing governments that struggled for control in Libya from 2014, each backed by loose alliances of armed groups.

A United Nations-backed unity government arrived in Tripoli last month and is attempting to establish its authority over the North African OPEC state, but it has continued to face vocal opposition from some in the east.

Libya's oil output has dropped sharply amid the political chaos, labour unrest and insecurity that have followed the uprising against autocrat Muammar Gaddafi five years ago.

Production is currently less than a quarter of a 2011 high of 1.6 million bpd.

The eastern NOC has long said it plans to export oil independently, but until last month had failed to secure a tanker.

After it shipped a cargo of oil on the Indian-flagged Distya Ameya on April 25, the United Nations blacklisted the vessel and it returned to a port in western Libya controlled by the Tripoli NOC.

Eastern officials have issued a series of defiant statements in recent days, vowing to continue their efforts to export oil.

(Additional reporting by Ahmad Ghaddar and Libby George in London; Writing by Aidan Lewis; Editing by Patrick Markey and Gareth Jones) ((pat.markey@thomsonreuters.com; +213-661-692993; Reuters Messaging: pat.markey.thomsonreuters.com@reuters.net))

Keywords: LIBYA SECURITY/TANKER