Rio, Fortescue cut iron ore output as China weakens

* Rio cuts iron ore output 10 percent as China weakens

* BHP sees no reason to make cuts; analysts wonder when

* Impact of China stimulus plan on steel making unknown

(Adds details, analysts' comments, updates shares to close)

By James Regan

SYDNEY (Reuters) - Rio Tinto Ltd/Plc (RIO.AX), the world's No. 2 iron ore miner, will slash output by as much as a third for the rest of this year, joining its Brazilian rival in trying to stem a fall in prices as Chinese steel demand slumps.

With the onset of a likely global recession cutting deep into steel use around the world, miners including Rio and Brazil's Vale (VALE5.SA)RIO.N have committed to cutting output by as much as 52 million tonnes, or about 6 percent of globally traded supply.

It remains to be seen whether those curbs will be enough to halt a collapse in benchmark Chinese spot iron ore prices, which have fallen from a record high of $197.50 a tonne in March to $63.50 last week, according to data from the Metal Bulletin trade journal. Spot prices have fallen below annual contract prices for the first time in years.

"There's more pain to come," said Australia & New Zealand Bank chief commodities strategist Mark Pervan. "China's largest iron ore port normally receives 10 vessels a week and they are currently receiving only four."