(Adds details, tables) TOKYO, Dec 28 (Reuters) - Japan's top oil refiner JX Nippon Oil & Energy Corp said on Monday it would trim its domestic crude refining plans in January from a year earlier, due to unspecified problems with its refining operations and scheduled maintenance.

The company, the core business unit of JX Holdings Inc , said it would process 1.11 million barrels per day (5.49 million kilolitres) of crude oil in January for domestic consumption, down 2 percent from a year earlier.

Japan's domestic demand has been easing due to a shrinking population, while a decline in kerosene demand accelerated this month due to warmer temperatures, a spokesman said.

JX's December crude refining for domestic consumption was estimated at 1.09 million bpd (5.35 million kl), down 5 percent from the year-earlier period, and slightly below its original plan of 1.11 million bpd (5.45 million kl) due to some problems with secondary units, a second spokesman said.

The refiner, which has 1.43 million bpd of crude refining capacity, or 36 percent of Japan's total, is scheduled to shut the 127,000-bpd No.4 crude distillation unit (CDU) at its Marifu refinery in western Japan from Jan. 24 to March 11 for planned maintenance. There was no maintenance in January this year.

Japan's JX Holdings and TonenGeneral Sekiyu have agreed to merge in 2017, joining forces to create a dominant player in a refining market that is in long-term decline.

Following are details of the company's exports and imports. Volumes are in bpd: Oil product exports Volume yr/yr (%) Dec 135,941 18 Jan 123,767 17 Oil product imports Volume yr/yr (%) Dec 54,782 50 Jan n/a n/a Following is a table of JX's estimates for Japan's domestic oil product sales this month. Volumes are in bpd: Product Volume yr/yr (%) Gasoline 979,991 -3 Kerosene 513,329 -18 Gas oil 618,835 -1 A fuel oil 245,505 -10 C fuel oil (utilities) 133,912 -29 C fuel oil (other) 105,506 -18 (Reporting by Osamu Tsukimori; Editing by Richard Pullin)