NEW YORK/TOKYO >> The yen jumped against the dollar late today, with traders on high alert for signs of intervention by Japanese monetary authorities to boost a currency languishing near 34-year lows.

The dollar fell sharply to 153 yen from about 157.55 yen for reasons that were not immediately clear, after the U.S. stock market closed and the Federal Reserve’s monetary policy meeting ended hours earlier.

The dollar was last at 154.85 yen.

The yen had rallied on Monday, with money market data suggesting Japan’s finance ministry had spent around $35 billion to prop up the currency that day. Official intervention data for the period will be announced at the end of May.

Traders have been on watch for weeks for possible intervention by Japanese officials, as even a historic exit from negative rates has failed to lift the currency.

When contacted by Reuters, Japan’s vice finance minister for international affairs, Masato Kanda, who oversees currency policy, said he had nothing to say about whether Japan had intervened in the market. Two days ago, he warned that authorities were ready to deal with foreign exchange matters “24 hours,” whether during London, New York or Wellington hours.

A U.S. Treasury spokesperson declined to comment on the yen/dollar move.

“It looked like intervention and it caught people by surprise,” said Marc Chandler, chief market strategist, at Bannockburn Global Forex.