TOKYO (Reuters) - Japanese authorities are ready to act against excessive yen rises, the government's top spokesman said on Friday, issuing a fresh warning to markets against recent yen gains that could hurt the country's export-reliant economy.

The dollar fell to a nearly four-week low of 100.10 yen on Thursday after the U.S. Federal Reserve trimmed its long-term interest rate expectations and the Bank of Japan rebooted its monetary policy framework.

"We're concerned about recent extremely nervous moves in the currency market," Chief Cabinet Secretary Yoshihide Suga told a regular news conference, when asked about the yen's recent rise against the dollar.

"The government hopes to keep watching currency market moves ever more carefully and if such moves persist, will be ready to take necessary action," he said.

Economy Minister Nobuteru Ishihara also told reporters on Friday he wants to closely watch yen moves and analyze its potential economic impact.

Japanese policy-makers say they do not have a specific level they have in mind as they try to talk the market out of pushing up the yen, but many traders see 100 to the dollar as authorities' line in the sand.

(Reporting by Leika Kihara and Kaori Kaneko; Editing by Simon Cameron-Moore)