U.S. President George W. Bush said Tuesday he was "troubled" by the movement of extremists from Pakistan to Afghanistan and would discuss the threat with Pakistan's Prime Minister here this month. Speaking at a White House press conference, Bush also said the United States would investigate charges by Afghan President Hamid Karzai that elements of the Pakistani intelligence services had been involved in attacks in Afghanistan. "We'll investigate his charge and we'll work with his service to get to the bottom of his allegation," the president said.