The yen's recent appreciation comes as the world's third-largest economy has been bottoming out from its deepest postwar slump, with authorities juggling a restart of economic activity with efforts to prevent a second wave of coronavirus infections.

The currency had been stable at around 107 yen to the dollar under Prime Minister Shinzo Abe administration, Aso told reporters after a cabinet meeting.

"Stability is important, so I'm closely monitoring it with a sense of urgency."

The dollar hit a 4-1/2-month low of 104.195 yen on Friday as investors worried that a recovery in the U.S. economy could be stymied by a second wave of coronavirus.

Japanese shares closed lower on Friday as the safe-haven yen strengthened on dismal U.S. data, while the resurgence of COVID-19 cases dampened hopes of a swift economic rebound, prompting authorities to discuss a response to market moves.

"The government and the Bank of Japan will keep a close watch on underlying market and economic trends and tackle as one as needed," Kenji Okamura, vice finance minister for international affairs, told reporters after a routine meeting with officials from the central bank and the Financial Services Agency.

A Japanese government panel acknowledged on Thursday that the economy peaked in October 2018 and fell into recession, suggesting it was struggling long before its more recent coronavirus slump.

"We have taken steps to support domestic demand rather than external, helping employment and household income improve, which led to a moderate economic recovery," Aso said.

Aso said exports accounted for less than 20% of Japan's economy, shrugging off any immediate impact from the yen's rise but the fact that he warned against the currency's gains underscored authorities' struggle to boost external demand.

Japanese authorities tend to fire warning shots against excessive currency volatility and disorderly moves at a time of market strains.

Tokyo has stayed out of the currency market since 2011 when it intervened heavily by selling the yen to prevent a strong currency from hurting an economy facing damage from a devastating earthquake, tsunami and nuclear disaster.

(Reporting by Tetsushi Kajimoto; editing by Chang-Ran Kim and Richard Pullin)

By Tetsushi Kajimoto