MANILA, JULY 6 -- Communist guerrillas said today that they would release "within the month" a U.S. Peace Corps worker and a Japanese aid worker being held captive but warned the Philippine military that any rescue attempt "might endanger their lives."

Speaking to journalists by telephone from the central Philippines island of Negros, a rebel spokesman identified as Ka (Comrade) Efren of the Negros Regional Command said the guerrillas had abducted the two foreigners because they were assisting the government's "anti-insurgency operation." Efren said the rebels would release them after completing an investigation into their activities.

"We did not harm them because we believe they were not conscious of what they were involved in," the rebel spokesman said. "Even if they are guilty, we will spare them this time. But we will not be so kind the next time." He also warned that "an offensive by the enemy might endanger their lives."

Fumio Mizuno of the Organization for Industrial, Spiritual and Cultural Development, a private Japanese organization, was abducted May 29 while visiting a silkworm farm near Bacolod, the provincial capital of Negros Occidental. Timothy Swanson, 26, of Cheyenne, Wyo., was abducted June 13 by armed men who came to his door in Patag village, also on Negros.

U.S. and Philippine officials had blamed the Communist New People's Army for both abductions, but until today's statement the rebels had not accepted responsibility. Diplomatic analysts said the rebel statement today appeared genuine.

The analysts also said the rebel spokesman's remarks lacked anti-American invective common in guerrilla statements, suggesting that the national Communist Party leadership may be unhappy over the abductions and that the regional rebel command is looking for a face-saving way to free the foreigners.

U.S. officials say they have not had direct contact with the kidnappers.

Swanson's abduction was discovered June 27, 14 days after he was taken and after the U.S. Embassy in Manila had ordered the evacuation of all 261 Peace Corps workers from the Philippines and suspended the program here indefinitely. Swanson's wife, Merle, said that when her husband was abducted, she did not tell anyone immediately on instructions from the guerrillas, who assured her that he would be returned safely.

U.S. officials here and in Washington said they ordered the pullout of volunteers after receiving intelligence reports that they might become targets of guerrilla attacks. One diplomatic source said U.S. officials became concerned after learning that the rebels had obtained a list of all Peace Corps workers in the country from the government's Economic Development Agency.

Diplomatic sources here said the Peace Corps program would likely remain suspended here at least until after the start of negotiations on the future of U.S. military bases in August. Before talks in May, the Communists, who are demanding an immediate U.S. military pullout, assassinated two American airmen outside Clark Air Base.

After those slayings, the New People's Army issued a statement warning that the U.S. bases and American military personnel were all potential targets for attacks.

Diplomats here said that while they never received any specific threat against individual Peace Corps workers, a recent Communist Central Committee directive expanded the list of potential U.S. targets to include the volunteers.

The rebel leader said Swanson "is being treated with respect." But he warned: "Maybe parents of young Americans should learn their lesson. We like Americans as long as they don't cooperate with their government's dirty tricks."

The Peace Corps pullout was seen as an embarrassment to President Corazon Aquino, who has said foreigners are safe in the Philippines. Aquino has been trying to project an image of political stability.