Microsoft has extended a search revenue guarantee agreement with Yahoo for one more year, amid reports that the Internet company is trying to break its 10-year agreement with Microsoft. The Redmond, Wash., software company, has agreed to extend the guarantee for an additional 12 months starting April 1, 2013, but only in the U.S., Yahoo said in a regulatory filing on Tuesday. Microsoft had in 2011 agreed to extend the guarantee in the U.S. and Canada through March 2013.