US STOCKS-Wall St set to tumble at open as investors flee risk
* Oil slump, Yellen comments stoke worries
* Dow futures fell over 300 points at one time
* Cisco up after results; only gainer on Dow premarket
* Futures down: Dow 246 pts, S&P 29 pts, Nasdaq 64.25 pts (Adds details, comment, updates prices)
By Abhiram Nandakumar
Feb 11 (Reuters) - U.S. stock indexes were set to open sharply lower on Thursday, with investors piling into safe haven assets as another fall in oil prices and cautious comments from Federal Reserve Chair Janet Yellen rekindled doubts about global economic health.
Yellen on Wednesday acknowledged tightening financial conditions and uncertainty about China and the risks that posed to the U.S. economy, but still kept open the possibility of further interest rate hikes.
Globally, stocks fell sharply on Thursday. The dollar hit a 16-month low against the yen as investors migrated to gold and top-rated bonds. U.S. Treasury security yields plunged to levels not seen since 2012 in some cases.
Prices on fed fund futures, used to predict future policy rates by the Fed, surged as investors further cut back expectations of another rate hike anytime soon after the first U.S. rate hike in nearly a decade in December. Continuación...