UPDATE 1-Arch Coal loss bigger-than-expected due to weak sales

* Fourth-quarter adjusted loss $0.45/share vs est. $0.39

* Revenue falls 17 pct to $719.4 mln

* Shares down 3.5 pct before the bell

Feb 4 (Reuters) - U.S. coal miner Arch Coal Inc reported a bigger-than-expected quarterly loss due to lower shipments and weak prices of both thermal and metallurgical coal.

Arch Coal, whose shares fell 3.5 percent in premarket trading, said it expects to sell between 131.5 million and 142.5 million tons of coal in 2014.

The company sold 139.6 million tons last year.

Arch Coal said rail service disruptions in the fourth quarter hurt shipments of thermal coal, used to generate electricity, from its Powder River Basin mines in Wyoming. The company did not specify the cause for the disruptions.

Total sales volumes fell nearly 16 percent in the quarter ended Dec. 31, from the third quarter.

Thermal coal prices have been depressed with U.S. power producers switching to natural gas, while a drop in demand for steel has hammered prices for higher-margin metallurgical, or steelmaking coal.

The weak prices led to Arch Coal posting a loss for the eighth quarter in a row.

Net loss widened to $371.2 million, or $1.75 per share, from $295.4 million, or $1.39, a year ago.

On an adjusted basis, the loss was 45 cents per share.

Analysts on average had expected a loss of 39 cents per share, according to Thomson Reuters I/B/E/S.

Revenue fell 17 percent to $719.4 million, below the average analyst estimate of $764.44 million.

Arch Coal shares were trading at $3.87 before the bell. The stock has fallen 43 percent in the past 12 months through Monday's close of $4.01.