* Japan intervenes for first time in six years
* BOJ continues to buy dollars in European session
* Dollar rises to 85.53 yen, runs into strong offers
* Key Ichimoku resistance at 86.40 untested
(Adds quote, updates throughout)
By Neal Armstrong
LONDON, Sept 15 (Reuters) - The dollar jumped more than two yen from a 15-year low after Japan intervened to sell yen for the first time in six years, but with key chart levels yet to break, traders were sceptical the impact would be sustained.
The intervention helped propel the euro, Australian dollar and sterling sharply higher on the day against the Japanese currency, although traders doubted Japan had bought anything other than dollars for yen.
Finance Minister Yoshihiko Noda said Japan intervened in the currency market as the impact of the yen's rise on the economy could not be ignored and that Japan would continue to take action, but that it had been acting alone.
The Bank of Japan acts for the Ministry of Finance in intervention, which traders said continued in the Asian and European sessions after an initial bout at around 83 yen per dollar shortly after the dollar hit a 15-year low of 82.87 yen.
At 0939 GMT, the dollar was up 2.2 percent at 85.21 yen JPY=, close to a session high of 85.53 hit on trading platform EBS. The euro was 2.5 percent higher at 110.54 yen EURJPY=R after briefly entering its Ichimoku Cloud, an indicator used to gauge momentum and future areas of support and resistance, at 111.01.
"I think we're now going to see persistent official buying of dollar/yen in the near-term," said Adam Cole, head of currency strategy at RBC Capital Markets.
Traders said official intervention persisted into the European session, with buying of dollars reported in the 84.95/85.00 area and at 85.25 on trading platform EBS, before strong offers above 85.50 capped the move.
"The rally will probably stall in the high 80s unless the U.S. economy bounces back or the Fed switches to a more hawkish policy. Until that happens, I dont think the gains in the dollar/yen can be sustained," said Lee Hardman, currency economist at Bank of Tokyo Mitsubishi-UFJ in London.
Some stop-losses were reported at 85.75, but technical analysts said the focus remained on the downside while dollar/yen held below key resistance at 86.40, which is the bottom of the closely watched Ichimoku Cloud.
Implied dollar/yen volatility was higher with the one-month JPY1MO= trading around 12.50 percent up from 12.3 on Tuesday but well below year-to-date highs seen in May near 18.00.
Risk-reversals, the premium required to hold a put or a call in a currency, were showing less of a bias for yen calls. The one-month 25-delta stood around 0.70 for yen calls compared with 1.50 on Tuesday JPY1MRR=ICAP <^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
For dollar/yen correlations link.reuters.com/wyn43p
For PDF on the yen's rise r.reuters.com/zuz33p
For graphic on intervention link.reuters.com/qep63p
For Reuters Insider on yen link.reuters.com/sav63p
The Bank of Japan started buying the dollar from around 10:30 a.m. (0130 GMT) on Wednesday.
Sources familiar with the matter said the central bank was ready to leave the intervention unsterilised -- holding off draining the funds that went into the currency market through the yen selling. [ID:nTKX006996]
The action was Japan's first currency market intervention since March 2004, when it halted a 15-month campaign to sell the yen after selling 35 trillion yen. Traders cited market estimates the latest intervention amounted to around 1.5 trillion yen ($17.67 billion)
Noda said Japan would take decisive steps if necessary, including intervention, and watch currency market moves closely.
Analysts said the impact of intervention was likely to diminish after speculative long yen positions had been eroded.
"Speculators have been long of yen so there is scope for further yen selling. But there's scepticism over whether the Japanese can change the trend as fundamentals haven't altered," said Beat Siegenthaler, fx strategist at UBS.
The yen gained fresh momentum on Tuesday after Japanese Prime Minister Naoto Kan won a leadership ballot against a rival seen as more willing to intervene to weaken the currency.
The dollar was down slightly against the euro at $1.2980 EUR= with traders reporting German selling. Good bids were highlighted at $1.2960 and $1.2940.
(Additional reporting by TOKYO markets team; editing by Nigel Stephenson)