* 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.
"I think the biggest theme is breaking away from central banks," said Jack Ablin, chief investment officer at BMO Private Bank in Chicago.
"The market is transitioning and attempting to stand on its own two feet. Central banks in general have been the market's protectors over the last 8 years."
At 8:32 a.m. ET, Dow e-minis were down 246 points, or 1.55 percent, with 83,909 contracts changing hands.
S&P 500 e-minis were down 29 points, or 1.57 percent, with 570,234 contracts traded. Nasdaq 100 e-minis were down 64.25 points, or 1.62 percent, on volume of 79,577 contracts.
At one point Dow futures were down more than 300 points, following the weakness in Europe and Asia. "300 as Europe" became one of the top 10 trending items on Twitter in the United States.
Wall Street, which had traded higher for much of the day on Wednesday after Yellen's comments, ended flat as a drop in oil prices weighed on energy and material stocks.
Oil prices were down yet again on Thursday, with U.S. crude tumbling 2.5 percent and Brent down nearly 2 percent.
Shares of Twitter were down 6.8 percent at $13.96 premarket after the company reported its first quarter with no increase in users since it went public.
Cisco was up 5.5 percent at $23.75 after reporting a bigger-than-expected profit.
All other Dow components trading premarket were lower, led by a 2.6 percent fall in JPMorgan.
Mylan was the biggest loser on the S&P 500 premarket, falling 12.7 percent to $44.10 after saying it would acquire Meda in a $7.2 billion deal.
The top gainers were travel companies Expedia, up 9 percent and TripAdvisor, up 11.3 percent, after their quarterly reports.
Another bright spot was Tesla, which said it expects to become profitable in 2016. The stock, which rose more than 10 percent after-hours on Wednesday, was up 3.7 percent at $149 premarket on Thursday.
Yellen is scheduled to resume her testimony before Congress at 10:00 a.m ET (1500 GMT). (Reporting by Abhiram Nandakumar in Bengaluru; Editing by Savio D'Souza)