LONDON, July 2 (Reuters) - The yen jumped suddenly against the dollar on Thursday, with traders alert to the prospect of intervention from Japan to prop up its stubbornly weak currency.

The dollar fell by as much as 0.9% to 161.115 JPY= and was last trading at 161.75.

It was not immediately clear what drove the market move or whether Japanese authorities were in the market. The move was smaller than that after previous bouts of intervention.

The yen crossed beyond the 162 per dollar level on June 30 to its weakest in 40 years, pressured by Japan's relatively low interest rates.

It received little help from a well-telegraphed Bank of Japan rate hike and an interim ceasefire between the U.S. and Iran, and no lasting boost from more than $70 billion in dollar-selling intervention in April and May.