An internal advisory for Citibank employees and clients cited law enforcement authorities when it warned that a Muslim extremist with al Qaeda ties had traveled to Bangkok from Thailand’s predominantly Muslim south and may be planning a terrorist attack. The warning focused on the Loy Krathong festival, one of Thailand’s biggest of the year, and the dates Nov. 17-21, but also warned against visiting Bangkok’s popular nightspots of Patpong, Nana Plaza and Soi Cowboy, or other places where Westerners congregate. Authorities said, however, that Thailand’s security forces were already on high alert during the year-end peak tourist season, and were doubtful that such a terrorist attack could occur.