Japan kept up verbal threats of further yen-selling interventions to weaken the unit on Tuesday, with its finance minister saying Tokyo is in a "war of nerves" against speculators.
Jun Azumi said he would make "appropriate decisions" on the need for additional market action if speculators try to push the yen higher.
Monday's yen-selling intervention came after the unit hit a fresh post-World War II high versus the dollar. Tokyo blames the rise on "speculators" and says the yen's current levels do not reflect Japan's economic reality.
"We are waging a war of nerves" with the currency market, Azumi said at a regular news conference.
"We will closely monitor developments (in the currency market) and make appropriate decisions at appropriate times."
Japan has been under domestic pressure to act as the strong currency erodes exporters' repatriated profits and makes domestically-produced goods less competitive, undermining a recovery from the March earthquake and tsunami.
Monday's intervention was Japan's fourth in just over a year, helping the dollar rise by roughly four yen to as high as 79.55 yen before easing back. On Tuesday the greenback was in the lower 78 yen range.
Traders estimate the latest intervention involved dumping a single-day record of about seven trillion yen ($92 billion) for dollars, according to Dow Jones Newswires.
The move came days before this week's meeting of leaders of the Group of 20 industrialised and emerging economies in France and may raise questions from Japan's global trade partners.
There are concerns that Tokyo's unilateral action could create friction between G20 nations, given that the United States has accused China of manipulating its currency to help inflate its exports.
Azumi on Tuesday said the action was in line with principles of international currency policy, saying it was intended to counter what he called "excessive" and "disorderly" moves in the exchange rate.
Tokyo has intervened unilaterally three times since September 2010, and also in March in coordination with its G7 allies in the wake of the earthquake and tsunami, but each time the initial impact has quickly worn off.
Japan's manufacturers have staged a rebound since the March disasters that left around 20,000 dead or missing, triggered a nuclear crisis and shattered crucial supply chains, heavily disrupting Japanese industry.
But fears have mounted that such efforts are being compromised by the strength of the Japanese currency, with every one yen rise wiping tens of billions of yen from the annual operating profit of giants such as Toyota.
This could force more firms to shift jobs and production abroad in what would be a further blow to the nation's slow economy.
The yen has hit post-war highs against the dollar and surged versus the euro as investors seek refuge from volatile markets stirred by eurozone debt fears and concerns for the global economy.
Analysts say Tokyo's attempts to weaken the unit are essentially futile in the face of a dollar whose weakness is beyond its control, while the yen's safe-haven status is undimmed by Japan's own economic problems.
There are calls for the Bank of Japan to adopt more aggressive easing to put further downward pressure on the yen, after last week's expansion of an asset buying scheme drew criticism for not being assertive enough.