Fears rose Wednesday that Russian President Vladimir Putin is intent on invading Ukraine, as U.S. and NATO talks with Russian officials failed to produce a commitment from Moscow to draw down its major troop buildup along the border with the developing U.S.-aligned democracy. Deputy Secretary of State Wendy R. Sherman, head of the American delegation, told reporters that U.S. and NATO officials stood firm in rejecting key Russian security demands for easing tensions over Ukraine, but left open the possibility of further talks with Moscow.