* China accuses mega miners of holding back cargoes to boost prices
* BHP says sells large volumes on spot market
* Rio Tinto declines comment on price manipulation claims
MELBOURNE, March 7 (Reuters) - BHP Billiton said on Thursday it was committed to a transparent iron ore market, responding to allegations by top consumer China that major miners had manipulated the market to drive an 80 percent jump in prices over the past six months.
China's national planning agency said on Wednesday the world's top three miners and some traders had delayed shipments and held back stocks "to send a fake market signal that there was a supply shortage".
BHP was the first of the iron ore giants to issue a response to the unusual statement from China's National Reform and Development Commission (NDRC), which came just as iron ore prices have eased off 16-month highs.
The NDRC did not name any miners, but the top three iron ore producers are BHP, fellow Australian miner Rio Tinto and Vale of Brazil. Vale was not immediately available for comment and Rio Tinto declined to comment.
BHP, the world's third-largest iron ore miner, said it had produced iron ore at full capacity between July and December 2012 and sold all of that material.
"We aim to improve transparency by increasing liquidity in the spot market," BHP said in a statement e-mailed to Reuters.
"We sell significant volumes on a spot basis, including through widely accessible trading platforms, irrespective of the iron ore price," it said.
Spot market trades are used to set index prices, which producers and steel mills in turn use as benchmarks for pricing monthly and quarterly iron ore supply contracts.
"It's more the art of war than manipulation," said a commodities analyst familiar with the way Australian iron ore miners price cargoes to China.
"Both sides use the pricing mechanism to their advantage and it's a big money game. If you can extract another 10 dollars per tonne out of your ore, that's a significant margin to make."
In response to the planning agency's specific allegation that some miners were buying iron ore cargoes on the spot market in order to lift prices of the key steelmaking material, BHP said it was "very normal" for steel mills, traders and producers "to both buy and sell cargo to balance their books".
"Such transactions occurring on the platforms is to the benefit of all market participants in that it supports transparent market pricing and market liquidity," BHP said.
BHP bought 100,000 tonnes of iron ore in January in a rare move that market participants saw as a strategy by producers to stem a decline in prices.
"We will not comment on what transaction we have or have not done," the company said on Thursday.
The attack from the NDRC comes at a time when steelmakers in China, the world's largest iron ore importer, are booking losses as steel demand has fallen and raw material costs have surged.
BHP shares were unchanged at A$35.82, while Rio Tinto's shares rose 0.6 percent in a weaker broader market.