HRABOVE, Ukraine — Ukraine accused pro-Russian separatists of shooting down a Malaysian jetliner with 298 people aboard Thursday, sharply escalating the crisis and threatening to draw both East and West deeper into the conflict. The rebels denied downing the aircraft. American intelligence authorities believe a surface-to-air missile brought the plane down but were still working on who fired the missile and whether it came from the Russian or Ukrainian side of the border, a U.S. official said.