The U.S. State Department issued a travel alert for Europe on Sunday, urging Americans to exercise vigilance about possible terrorist attacks. But the agency stopped short of issuing a travel warning, a stronger step that would have had immediate implications for thousands of American students in Europe. Many study-abroad programs will not send students to areas for which a warning is in place, for safety and liability reasons. In a conference call with reporters on Sunday, Under Secretary of State Patrick F. Kennedy said emphatically that study-abroad programs should not be canceled in response to the alert. “We’re not recommending that American citizens … should defer travel to Europe at this time,” he said. He did, however, urge Americans in Europe to be aware of safety measures and to register with the local consulate or embassy in case of an emergency.