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The Nikkei share average fell 395.04 points to 14,420.94 in mid-morning trade after dropping to a low of 14,408.62 earlier, the weakest since February 17.
"Investors are unwinding their long positions in the Nikkei and short positions in the yen," said Kyoya Okazawa, head of global equities and commodity derivatives at BNP Paribas. "Short-term sellers like commodity trading advisors are also big players today and they are also reacting to the falling copper price."
On Thursday, copper resumed its decline on the London Metal Exchange after a brief recovery on the previous day. It hit a 44-month low on Wednesday.
Nonferrous metal shares, which have been hit over the past few sessions, extended their declines. Dowa Holdings dropped 3.0% to a nine-month low of 807 yen, and Sumitomo Metal Mining shed 2.0% to a five-week low of 1,240 yen.
Traders said investors remained risk-averse amid the backdrop of troubles in Ukraine and soft data in China, adding that the benchmark Nikkei would likely target support at 14,000 if it breaches Friday's futures and options settlement price of 14,429.87.
Market participants, citing estimates by local brokerages, said that Nikkei futures and options contracts expiring in March likely settled at 14,429.87. The official settlement price will be announced by the Osaka Securities Exchange after the market closes.
Markets remained nervous on rising tensions in Ukraine after Russia launched military exercises near its border with Ukraine.
Index heavyweight stocks led the declines. SoftBank Corp fell 1.8% and was the most trade stock by turnover, while Fast Retailing Co declined 2.7% and was the second-most traded stock.
Exporters were also battered after the dollar/yen hit the lowest in over a week at 101.83 yen. A weak yen hurts Japanese exporters' competitiveness abroad as well as their dollar earnings when repatriated.
Toyota Motor Corp slid 2.3%, Nikon Corp tumbled 4.1% and Tokyo Electron Ltd shed 4.2%.
The Topix dropped 2.4% to 1,174.83, with all of its 33 subsectors in negative territory.
The JPX-Nikkei Index 400 , a gauge comprising firms with high return on equity and strong corporate governance, dropped 2.4% to 10,624.75.
