For more than a year, Washington has been pressing Syria to seal its border with Iraq. Two months ago, senior American officials delivered that message to President Bashar Assad in Damascus and expressed concern over Syria's longtime support of anti-Israeli terrorist groups. William Burns, a State Department official who was accompanied by Peter Rodman of the Defense Department later said that Assad was told that 'Syria should not be used as a platform to undermine Iraqi stability.' In an interview, a senior Defense Department official complained that 'elements in the Syrian' government 'are actively colluding with our enemies.' He says that 'extremists in Iraq are using Syria as a place to organize and to get support and to flow back and forth across the border, and we believe this is tolerated by the Syrian government. . . . This means they share responsibility for the killing of Americans, and this has to stop.'