The U.S. military is offering to evacuate the families of American servicemen at government expense following the recent slayings of Americans and rebel threats of more attacks. A military spokesman said today that U.S. service personnel have until Thursday to tell officials whether they want their families sent home. ``We have asked those who have families to tell us when they would like to ship their dependents back before their tour ends,'' said Lt. Col. Ronald Rand, spokesman for Clark Air Base, 50 miles north of Manila. Rand did not say what would happen if service members decided later to return their dependents. Adm. Huntington Hardisty, commander of U.S. forces in the Pacific, authorized a program which allows a serviceman at any of the six U.S. bases in the Philippines to send members of his family home at government expense before his tour of duty ends, Rand said. Rand said a U.S. government-sponsored transfer of dependents of U.S. servicemen is usually done when only on exceptional cases, such as illness. But he said a large number of requests and letters of concern about their families forced them to adopt the program. A total of 40,000 Americans _ servicemen, civilian workers and dependents _ are stationed at Clark, Subic and four other American bases in the Philippines. At Clark alone, there are over 10,000 dependents, Rand said. Last month, Communist rebels killed two American airmen near Clark and vowed more attacks until all U.S. troops leave the country. Rebels also are suspected in the May 4 shooting death of a Marine sergeant near Subic. At least eight Americans have been slain in political attacks since April 1989. Six of the victims were servicemen or associated with the military. Another American serviceman was killed Saturday night, but American and Filipino officials said robbery _ not politics _ was the motive this time. Since Friday, U.S. Defense Department civilians and military dependents who live on Clark have been barred from leaving the base because of Communist threats. Those living off-base have been ordered to limit travel to essential business. Rand said the restrictions would last for at least two months depending on U.S. assessment of the threat. The New People's Army, the military wing of the outlawed Communist Party of the Philippines, has waged a 21-year-old insurgency to establish a Marxist state.