Japan intervened in the currency markets yesterday to sell yen for the first time in six years and promised more to come in a bid to stop its relentless rise from threatening a fragile economic recovery.

Fresh after victory in a party leadership contest, Japan's Prime Minister Naoto Kan appeared to be stepping up efforts to wrench the country out of deflation by targeting yen strength, which has weighed on stock prices and corporate profits.

Kan said the intervention had had some effect but the government was watching foreign exchange moves with a sense of urgency.

Even as the dollar surged as much as 3% on the day against the yen, doubts about the ultimate effectiveness of Japan's unilateral yen selling spree abounded.

A 15-month solo effort by Switzerland to weaken its currency did little to tame the Swiss franc, and European Union officials waded in to say that co-ordinated action always proves more effective.

Aside from apparently acting alone, Japan faces the stiff task of trying to put a halt to yen strength while other major central banks such as the Federal Reserve may be considering additional steps to ease policy that could weigh on their respective currencies.

"It is far less clear that intervention will be effective in a world of zero interest rates and excess liquidity, but we think it still makes sense for Japan to try to arrest yen strength," said Richard Jerram, chief Asia economist at Macquarie Securities in Tokyo.