Japan stepped up warnings on Friday that it may intervene in the forex market if excessive yen rises continue, with the top currency diplomat of the country threatening action even if U.S. and European partners were not on board.

Vice finance minister for international affairs, Takehiko Nakao, said it was becoming more obvious that present yen rises were being driven by speculators and issued a warning that Tokyo was ready to intervene should such moves continue.

"Although there might be differences (in views with the United States and European countries) from time to time, the Japanese government is determined to take an immediate response to volatility in the currency market," he told a Euromoney forum.

"Monetary policy is important, but we shouldn't exclude the possibility of taking our response in the market, which is intervention," he said.

"It is clear that the current one-sided currency moves do not reflect the economy's fundamentals," Azumi told a news conference after a cabinet meeting.

"We will need to take decisive action if excessive currency moves continue."

The warnings came after the Japanese currency climbed to another 3-1/2 month high against the dollar of 78.21 overnight and an 11-1/2-year high against the euro of 96.48 on concerns of escalating euro zone debt crisis and weak U.S. economic data.