(Adds background on mine, share movement) Jan 18 (Reuters) -...

(Adds background on mine, share movement)

Jan 18 (Reuters) - Miner Whitehaven Coal (WHC) on Thursday reduced its full-year guidance for saleable coal output after a decline in production from one of its main mines in the December quarter due to work to fix unstable roof conditions.

Coal output for fiscal 2018 is now expected at 20.5 million tonnes to 21.0 million tonnes, on a 100 percent basis, down from a range of 22 million to 23 million tonnes forecast in August, the company said in its quarterly report.

Whitehaven logged saleable coal production of 5.0 million tonnes for the quarter, slightly lower than 5.1 million tonnes a year ago.

The company said that production at its Narrabri mine was affected by the installation of additional roof support during the quarter, which would also lead to a rise in operating costs of about A$2 a tonne ($1.59) over the mine's remaining life.

Saleable coal production for the quarter from the mine was 1.68 million tonnes, about 10 percent below a year earlier.

Whitehaven, however, added that the outlook for thermal coal in the short to medium term was "favorable".

Shares of the company, which have risen strongly over the past six months, fell 6.8 percent after the announcement to a near-four week low.

($1 = 1.2569 Australian dollars)