MANILA, Philippines (AP) _ The U.S. military has offered to ship the families of servicemen home following the recent slayings of Americans and threats by rebels to kill more, a military spokesman said today.

Also today, the U.S. Air Force began a two-week training exercise that American sources said was cut back because of the security threat.

Adm. Huntington Hardisty, commander of U.S. forces in the Pacific, has authorized a program for the voluntary return of dependents of U.S. servicemen stationed in the Philippines, said Lt. Col. Ronald Rand, spokesman for Clark Air Base, 50 miles north of Manila.

Under the program, a serviceman at any of the six U.S. bases in the Philippines may request to send members of his family home to the United States under U.S. government expense before his tour of duty ends, 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 shooting death of a Marine sergeant near Subic Bay last month.

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.

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.

The Philippine government has responded to the recent killings by increasing police protection around Clark Air Base in recent days.

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.

The restrictions affect about 20,300 Americans.

A total of 40,000 Americans - servicemen, civilian workers and dependents - are stationed at Clark, Subic and four other American bases in the Philippines.

About 1,000 Air Force personnel, including some from units based in Japan and South Korea, had been expected to take part in today's training operation, the seventh and last phase of the Cope Thunder exercise during this fiscal year.

But the number was cut back to about 850 so that all personnel could be housed at Clark Air Base, 50 miles north of Manila.

In Honolulu, Air Force spokesman Maj. James Pasierb claimed the reductions were not prompted by ''terroristic activity'' and would not affect the level of training.

However, the U.S. sources, who spoke on condition of anonymity, said security was indeed the primary consideration for the reductions. Clark is located in a stronghold of the New People's Army.

In the latest attack on Americans, a U.S. Navy supply officer was fatally stabbed Saturday night near Subic. The Navy identified the victim as Lt. Richard E. Brown, 33, of Spring Valley, N.Y.

Police said Brown's wallet and other valuables were missing. The Navy banned sailors from leaving the base after the killing but lifted the restrictions Sunday after deciding robbery and not politics was the motive.

During previous Cope Thunder exercises, hundreds of airmen on temporary duty were housed in civilian hotels near Clark because of a housing shortage on the base.

The policy was changed, however, after the rebels killed the two airmen on May 13, the day before U.S. and Philippine negotiators opened talks on the future of the U.S. bases.

After the killings, hundreds of airmen were moved back to Clark, and some were housed in the gym because of the housing shortage.