JAKARTA, Jan 20 (Reuters) - Indonesia's government has eased a coal export ban for 139 companies as of Thursday, a senior energy ministry official said, after the firms met local market sales requirements aimed at averting a supply crunch and power outrages.

The world's biggest thermal coal exporter on Jan. 1 imposed a month-long export ban without warning, causing jitters in markets and among major importers such as Japan, the Philippines and South Korea.

Authorities have started a calibrated easing for firms that meet a Domestic Market Obligation (DMO) that has been central to the high-profile suspension, which was introduced to ward off widespread power outages after local plants reported critically low coal inventories.

Authorities have blamed the coal supply saga on poor compliance of the DMO policy, under which coal miners must sell a quarter of their output to local buyers, with a $70 per tonne price cap for power generators.

"This export ban is temporary, it's emergency situation management to ensure domestic coal supply," Ridwan Djamaluddin, director general of minerals and coal at the energy ministry, told a media briefing

He said 75 ships had been allowed to load coal from firms that had met all their DMO requirements, while 12 more had been allowed to proceed having provided a written assurance of compliance and acceptance of penalties.

Another ministry official on Tuesday said coal stocks at local plants had improved and authorities had stepped up monitoring of deliveries to generators.

The ministry has set a target of enabling local power plants to have enough inventory by the end of January for more than 20 days of operations. (Reporting by Bernadette Christina Munthe; Writing by Martin Petty; editing by David Evans)