The Nikkei stock average plunged below the key 13,000 line Monday amid the dollar's sharp dive below ¥103 and speculation that the U.S. is poised to enter a recession.

At 5 p.m. Monday, the dollar traded at ¥102.68-70 in Tokyo, down from ¥104.33-35 at 5 p.m. Friday, marking its first drop into ¥102 territory since January 2005. As of 11 a.m. (GMT) in London, the yen dropped to 102.90-103.00 versus the dollar.

The strong yen trend, which is bad for Japan's exporters, triggered the 225-issue Nikkei average to drop 610.84 points, or 4.49 percent, to close at 12,992.18 — its lowest closing level since Jan. 23.