* Sees non-cash items for Minas Rio and some Australia coal assets
* Charge to be taken in first-half results due July 24
* Q2 diamond output down 6 pct, copper production off 5 pct
* Shares down 0.5 pct, not far from 12-year low (Adds analyst comment, share price)
By Eric Onstad and Clara Denina
LONDON, July 16 (Reuters) - Mining group Anglo American has warned of a second multi-billion dollar writedown this year on its coal and iron ore assets, demonstrating the growing impact of sliding commodity prices.
The charge of between $3 billion and $4 billion flagged on Thursday, to be taken in its first-half results, comes on top of a $3.9 billion writedown Anglo took for similar reasons in February, when it also posted a 25 percent drop in underlying operating profit for 2014.
Anglo, the fifth-biggest diversified global mining group by stock market capitalisation, is not alone in feeling the pinch of tumbling commodity prices.
BHP Billiton said on Wednesday it will take a $2 billion impairment on its U.S. shale operations, the third writedown in three years.
Anglo has been fighting the impact of struggling metals prices by trying to improve the efficiency of its mining operations and by selling less profitable assets, including coal mines in Australia.
Both writedowns this year involved its recently launched Minas Rio iron ore mine in Brazil, plagued by delays and cost overruns since Anglo bought it in 2007-2008 for about $5.5 billion.
"It underlines the fact that those assets are probably not going to make any money," said analyst Ben Davis at investment bank Liberum.
The price of iron ore .IO62-CNI=SI, Anglo's most profitable product last year, has nearly halved over the past 12 months. And coal has also been under pressure. Australian coal prices hit an eight-year low earlier this year, with prompt cargoes from the Newcastle terminal dipping to $58.55 a tonne in June.
"Anglo American expects to record non-cash impairments ... relating to Minas Rio and certain Australian coal assets of approximately $3 billion to $4 billion on a post-tax basis," it said.
Anglo, which reports first-half results on July 24, also said diamond output fell 6 percent at its De Beers unit.
Diamonds last year became the second-largest earner for the company and analysts say they may take the top place this year after sharp falls in other prices.
"I think the biggest element of disappointment was how weak the sales were in diamonds ... and that tells us that sales are probably going to get harder as conditions haven't improved," said Davis.
Anglo also said copper production in the second quarter had fallen by 5 percent, mainly due to production issues in Chile, while iron ore output in South Africa dropped 9 percent.
Anglo shares shed 0.5 percent to 870.25 pence by 0811 GMT, slightly underperforming a largely flat UK mining index . The stock earlier this month hit a low of 825p, its lowest in more than 12 years. (Additional reporting by Sarah McFarlane; Editing by Veronica Brown and David Holmes)