WASHINGTON, March 27 (Reuters) - Washington estimates up to 90 percent of suicide bombers in Iraq enter the country via Syria which has not acted to stop this flow of attackers, the U.S. State Department's Iraq adviser said on Tuesday. David Satterfield, who is Secretary of State Condoleezza Rice's key adviser on Iraq, said Syria had an obligation to stop "jihadists" crossing into Iraq where suicide bombings are an almost daily occurrence. He estimated 90 percent of suicide bombers in Iraq were foreigners and while the mix of nationalities changed, some 85 to 90 percent of them crossed over from...