Relatives of the Americans - four men and a woman - alerted the U.S. Coast Guard that the 38-foot catamaran The Pineapple left Guam on Jan. 6 but had not reached its destination on Cebu Island, said Philippine coast guard chief Adm. Wilfredo Tamayo.
CBS News' Barnaby Lo in Manila reports that one of the four men aboard the vessel is a member of the U.S. Air Force. Their boat should have reached the Filipino coast five days ago.
Lt. j.g. Justin Valentino, a search and rescue coordinator for the U.S. Coast Guard in Guam, said that while weather conditions make predictions difficult, the journey was supposed to take seven to 10 days.
Tamayo said the U.S. Coast Guard requested a search although it was not clear whether the sailboat had reached Philippine waters. The U.S. Coast Guard was sending an HC-130 Hercules to join the operation.
The Philippines dispatched two aircraft to scour the waters off central and southern islands, and coast guard detachments and passing ships were alerted to be on the lookout for the sailboat, Tamayo said.
Guam, a U.S. territory, is about 1,400 miles east of Cebu, where heavy rains and rough seas have been reported in recent weeks.
"The weather at that time was moderate or rough to very rough," Tamayo said.
Coast guard spokesman Cmdr. Armand Balilo said the five people included a Filipino-American in the U.S. Air Force. Their identities and hometowns have not been released.