(Adds company comment, details on costs) Feb 15 (Reuters) -...

(Adds company comment, details on costs)

Feb 15 (Reuters) - Australia's Whitehaven Coal (WHC) said on Friday its first-half profit rose 19 percent helped by robust prices for its higher grade thermal coal, but investors were rattled by increasing costs, knocking its shares lower.

Net profit for the six months ended Dec. 31 came in at A$305.8 million ($217.3 million), up from with A$256.2 million last year.

"Recently imposed coal import restrictions by China have led to reduced demand for Indonesian low CV (calorific value) coal and have helped to increase the price differential that exists between high and low CV coals," Whitehaven, which doesn't sell to China, said in a statement.

China cut coal imports last December following signals from Beijing that it would stop clearing shipments until 2019. Shipping data earlier this month showed dozens of ships carrying coal and iron ore to China stuck outside ports waiting to unload.

Whitehaven said revenue for the period rose 11 percent during the period to A$1.27 billion.

However, the cost of producing coal rose by A$12 a tonne for the period to A$69 a tonne, and it increased its full year cost guidance to A$67 a tonne from A$64 previously.

Costs have risen due to higher diesel prices, increased washing of coal to sell more higher quality coal and lower output from the company's lower cost mines, Whitehaven said.

The company also trimmed its fiscal 2019 guidance for saleable coal to 21.5 million tonnes to 22.5 million tonnes, down from an earlier 22-23 million tonnes as it changed its output mix to take advantage of solid prices for higher grade coal.

Whitehaven shares fell as much as 9.2 percent to a one-month low and were last trading down 5.6 percent at A$4.52 a share in a flat broader market.

The company declared an interim dividend of 15 cents a share, above last year's 13 cents a share, and said it would pay a special dividend of 5 cents a share. ($1 = 1.4075 Australian dollars)