The sources, one government and another a market source, spoke on condition of anonymity as they were not authorised to speak to the media.
The US dollar initially fell to 155.5 yen after the move, its lowest since March 2, in what would have been its largest single-day drop since late December 2024 if it had stayed at that level.
It regained some ground and was last down 2.38 per cent at 156.57 yen.
Before Thursday's action, investors had amassed the largest short yen position in nearly two years, selling the currency against the euro, Swiss franc, UK pound and Australian dollar on the view that neither rate hikes nor the threat of intervention would come to the currency's aid.
The bet was also the biggest since Japan last stepped into currency markets in 2024, setting up a fresh test of policy makers' resolve to curb yen speculation.
Japanese Finance Minister Satsuki Katayama said earlier on Thursday that the time to take "decisive action" in the market was nearing, in her strongest signal yet of potential currency intervention to prop up the sagging yen.
"I think that the market thought that it was just verbal intervention. It caught the market on the wrong side," Marc Chandler, chief market strategist at Bannockburn Capital Markets in New York, said.
"Intervention without a policy adjustment is not thought to be very effective. It will reinforce the sense that JPY 160 is a pain point for Japanese officials," he added.
Intervention is likely to be effective if Thursday's action clearly signals a broader policy shift, Chandler said.
The strongest message would be if authorities "then back up that message of material intervention with raising rates to make it more expensive to short the currency ... The BOJ's balance sheet will be scrutinised to see if material intervention did take place".
The Nikkei earlier, citing a government source, said officials had intervened by buying the currency, which was around its weakest since July 2024 earlier on Thursday.
Top currency diplomat Atsushi Mimura also said earlier the timing to take decisive action was approaching, adding that "extremely speculative" moves in the currency market were increasing.
The Ministry of Finance has threatened intervention in currency and oil markets and on Thursday, reiterated that action could be "on all fronts".
"This is our final evacuation warning to markets," Mimura told reporters.
When asked whether he was alluding to the chance of an imminent yen intervention, Mimura said: "I think market players would know what I mean."
