By Masayuki Kitano and Charlotte Cooper

TOKYO, July 9 - The yen has torn to its highest level against the dollar in five months and within sight of a 13-year high set in January, prompting speculation that Japan might try to curb its rise to prevent it damaging exports.

The yen hit 91.80 per dollar on trading platform EBS on Wednesday, leaving the dollar down 2 percent on the day. The dollar has firmed on Thursday to 93.30 yen but still stands 8 percent below a six-month high traded in April.

The yen also surged to its highest in two months against the euro, the Australian dollar and sterling. All three have recovered 2-3 yen on Thursday.

Japan's authorities intervened heavily earlier in the decade to stop a rising yen from harming its all-important exports.

But they haven't intervened since March 2004, the end of a 15-month long spree in which they sold 35 trillion yen ($546 billion) to protect growth.

WOULD THEY INTERVENE NOW?

Unlikely. The yen may have been on a tear in the last 24 hours, but it surged 27 percent against the dollar from a low in August to a high in January. If authorities didn't intervene then, they are unlikely to do so now.

Also, authorities may have felt more temptation to intervene late last year because exports were collapsing as the crisis weighed on global demand. Exports are still showing significant falls from a year earlier, but monthly changes suggest the worst of the slump is over.

The dollar fell to a 13-year low of 87.10 yen in January as the global economic crisis saw panicked investors repatriating funds, forcing liquidation of deeply embedded short-yen positions which had built up over years.

Even a green light to intervene from the Group of Seven industrial powers last October did not persuade Tokyo to intervene in currency markets.

At that time, the G7 issued a rare inter-meeting statement singling out yen volatility, which was interpreted as giving Japan the approval of its fellow big nations to stem the yen surge if it wanted to.

"Even when the dollar fell to 87 yen, there was not any sign that there might be intervention, so if you ask whether the probability will increase because of the fall to 92 yen from 95 yen, that does not seem to be the case," said Junya Tanase, foreign exchange strategist for JPMorgan Chase Bank in Tokyo.

"There has not been any intervention since 2004. The policy on intervention has probably changed."

WHAT DO THE OFFICIALS SAY?

The top government spokesman on Thursday repeated an often-used phrase that excessive currency moves were undesirable for the stability of the economy.

When they are particularly concerned, they tend to step up verbal warnings before any actual intervention, usually by saying that Japan will take decisive actions in markets, or something to that effect.

On May 22, when the yen was trading around 94 per dollar, Finance Minister Kaoru Yosano said he was not considering intervention.

SO IS INTERVENTION OUT OF THE QUESTION?

The possibility increases below 90 yen per dollar and the risk gets much higher at or below 87. But just a drop in the dollar is not necessarily enough to spark intervention.

The Ministry of Finance would be likely to watch the yen's exchange rate against other trading partners as well and keep its eye on the Nikkei share average as a barometer of the impact of a stronger yen on Japanese companies.

The Nikkei hit a 26-year closing low of 7,054.98 in March but closed on Thursday at 9,291.06, down 1.4 percent on the day but 32 percent above the March level.

"The simultaneous move is important - dollar/yen below 90, the Nikkei below 8,000, and the yen crosses all down are the necessary conditions ," said Masafumi Yamamoto, head of FX strategy Japan at Royal Bank of Scotland.

"Even if the dollar/yen falls below 90 and the Nikkei is above 9,000, there's not the case for intervention."

BUT THE SWISS NATIONAL BANK HAS INTERVENED, WHY NOT JAPAN?

As the world's second-largest economy, Japan would cause bigger trade tensions by trying to hold down the value of its currency than Switzerland, the world's 21st-largest economy.

The Bank of Japan has raised its assessment of the economy. While still deep in recession, economic conditions are not getting worse.

Tanase notes the yen's real effective exchange rate is around its long-term average, meaning it is not particularly strong at the moment.

"To stem a rise in the currency at a time when the yen's level is not strong would lead to maintaining competitiveness," Tanase said

"The chances are probably low that such action by Japan would be tolerated at a time when economic conditions in other countries are also very poor."

(Additional reporting by Hideyuki Sano; Editing by Neil Fullick)