UPDATE 7-U.S. says Iran nuclear talks to continue past deadline

* Tuesday midnight deadline for outline eludes negotiators * US says would leave talks before June 30 if unable to reach deal * U.N. sanctions, R&D after 10 years remain unresolved (adds remarks by State Dept, Iranian negotiator, White House, Fabius) By Parisa Hafezi, Louis Charbonneau and John Irish LAUSANNE, Switzerland, March 31 (Reuters) - The United States said on Tuesday that world powers and Iran would keep negotiating over an outline accord on curbing Tehran's nuclear programme beyond a