* Yellen: Risks "moderated, not elevated," no bubbles forming
* Fed's Lockhart calls middle-of-year rate hike appropriate
* Tech stocks weigh; Apple down for third session
* Indexes down: Dow 0.9 pct, S&P 0.8 pct, Nasdaq 0.8 pct (Updates to afternoon with extension of losses)
By Noel Randewich
May 6 (Reuters) - U.S. stocks added to losses on Wednesday afternoon and the Dow sank into negative territory for 2015 after U.S. Federal Reserve Chair Janet Yellen warned of high valuations, adding to anxiety about future interest rates and a global bond rout.
The S&P 500 dipped to its lowest since early April after Yellen said high equity valuations could pose dangers, although she also said she does not see any bubbles forming.
Atlanta Federal Reserve bank president Dennis Lockhart said he still expects it will be appropriate to raise interest rates some time in the middle of the year, and that market expectations of a September increase were in "reasonable alignment" with the central bank's likely path.
His and Yellen's comments came as investors try to pinpoint when the Fed will begin raising interest rates for the first time since 2006. An April payroll report later this week may affect when the Fed will make its move.
"Everyone is obsessed with the Fed," said Michael Church, president of Addison Capital Management in Philadelphia. "It shouldn't surprise anyone that we didn't break out to new highs this week, given that you had Yellen speaking today and payrolls coming out on Friday."
Most of Wall Street's top banks see the Federal Reserve holding off until at least September before raising interest rates, based on Reuters' most recent poll.
A worldwide selloff in government bonds was also spreading unease across Wall Street and other financial markets.
At 3:11 p.m., the Dow Jones industrial average fell 156.72 points, or 0.87 percent, to 17,771.48, the S&P 500 lost 17.02 points, or 0.81 percent, to 2,072.44 and the Nasdaq Composite dropped 39.24 points, or 0.79 percent, to 4,900.09.
Yellen's comments stung investors already nervous about stock prices. The S&P 500 currently trades at 16.8 times forward earnings, higher than its 10-year median of 14.7, according to Thomson Reuters StarMine.
With Wednesday's loss, the Dow was down 0.24 percent in 2015 while the S&P was up 0.68 percent.
The telecom services index was the biggest loser among the 10 major S&P sectors, down 1.7 percent.
Tech stocks, led by Apple and Microsoft, were the biggest drag on the three major indices. Apple was down 1.2 percent, declining for its third straight session.
MoneyGram gave up some of its earlier gains after Western Union said it was not in talks to buy the company. MoneyGram was up 16.54 percent while Western Union was up 4.29 percent.
Declining issues outnumbered advancing ones on the NYSE by 2,369 to 708, for a 3.35-to-1 ratio on the downside; on the Nasdaq, 1,693 issues fell and 1,039 advanced for a 1.63-to-1 ratio favoring decliners.
The S&P 500 was posting 6 new 52-week highs and 6 new lows; the Nasdaq Composite was recording 26 new highs and 69 new lows. (Reporting by Noel Randewich, additional reporting by Ann Saphir and Tanya Agrawal; Editing by Nick Zieminski)