TOKYO, July 25 (Reuters) - Japan's Nissan Motor slashed its annual outlook on Thursday after deep discounting in the United States almost completely wiped out the automaker's first-quarter profit.

The figures confounded the expectations of analysts who saw profit exceeding last year's, sending Nissan stock plunging 7%.

Investors now have to worry about Nissan's prospects in the United States, a fresh concern for an automaker already fighting to turn around its fortunes in another critical market, China.

Operating profit for April-June totalled 995 million yen ($6.5 million) versus 128.6 billion yen in the same period a year earlier. The result was just a sliver of the 164.4 billion yen average of five analyst estimates compiled by LSEG.

"The first quarter was a very tough one for Nissan," Chief Executive Makoto Uchida told an earnings briefing. "However, we'll recover our performance by taking clear measures to address the challenges and launching new models."

The automaker is "optimising inventory buildup" in the U.S. and will focus on the quality of sales, he said. It plans to bolster sales by offering new and refreshed models such as the Armada and Murano SUVs in the second half of the financial year.

Having endured its worst quarterly performance in over three years, the automaker cut its operating profit forecast for the financial year by 17% to 500 billion yen.

Nissan said U.S. sales were hurt by an ageing portfolio and a market shift to hybrid vehicles.

Ghosn led a stunning turn around to profitability starting in 1999. Nissan/Infiniti made their best, most competitive vehicles in the 2000's and early 2010's.