The U.S. economy should accelerate in the second quarter to an annual growth rate of 3 percent or more, but it may not be clear for some time if the pick-up is sustainable, a Federal Reserve official said on Sunday. Federal Reserve Bank of Atlanta president Dennis Lockhart also said he expected the U.S. central bank to have run down its bond-buying program by its October or December meeting and to start raising interest rates in the second half of next year. The U.S. economy hardly grew in the first quarter but has gathered pace since.