The Nikkei was down 1.3% at 63,360.96, as of 0046 GMT. Earlier in the session, the benchmark fell as much as nearly 3%, slipping below the 63,000 level for the first time since May 22. The broader Topix slid 1.6% to 3,787.08.
Washington carried out renewed strikes against multiple targets overnight in Iran, the U.S. military said on Wednesday, hours after President Donald Trump vowed new attacks if no peace deal is secured.
Tech investment conglomerate SoftBank Group dragged the Nikkei index by 233 points, down 3.6%. Fujikura, which produces optical fibre that is a key component of AI data centres, lost 6%, while semiconductor testing equipment maker Advantest shed 3.7%.
There were 184 decliners in the Nikkei index against 40 advancers.
