WASHINGTON -- The State Department issued a worldwide travel alert to American citizens on Monday night ahead of the Thanksgiving holiday, one of the busiest travel weeks of the year, faced with an increased threat from Islamist extremists affiliated with terrorist organizations.The alert specifically warned of the continued threat from Islamic State shortly after the group orchestrated a series of attacks across Paris. Shortly after the attack, Islamic State released videos threatening to attack Washington and New York.The video threat against New York featured stock images of Herald Square, known for its central role in the annual Thanksgiving Day parade. US authorities say there are no specific, credible threats against the homeland at this time. Nevertheless, "authorities believe the likelihood of terror attacks will continue as members of ISIL/Da’esh return from Syria and Iraq," the alert reads. "Exercise particular caution during the holiday season and at holiday festivals or events."The alert also asks citizens to avoid crowded places and to remain vigilant.