The secretive Islamic Jihad reiterated today a claim that it had executed kidnaped U.S. diplomat William Buckley and made a bid to swap his body for 100 Palestinians jailed in Israel.

The statement, delivered at midnight, was accompanied by two overexposed Polaroid photographs. The faded pictures showed the face and upper torso of a man cloaked with a sheet. The bearded figure bore a strong resemblance to Buckley, and reproductions of the snapshot offered the first possible evidence that the U.S. Embassy political officer kidnaped here last year may be dead.

"We assure the Moslem people of the world that we have executed the American spy in reply for the blow dealt to the Moslem people in Tunis," said the Islamic Jihad communique, distributed to the daily An Nahar and a western news agency.

Eight days ago, the same organization announced Buckley's execution in retaliation for an Israeli raid against the Palestine Liberation Organization headquarters south of Tunis. The first claim was accompanied by a picture showing Buckley still alive. The communique, delivered to the press last week in the midst of a crisis over the kidnaping of Soviet Embassy personnel and the slaying of a Soviet diplomat, had left open the question whether Buckley actually was dead.

In Washington, a State Department spokesman said that the United States does not regard the photograph purported to be of Buckley "as convincing evidence that Mr. Buckley is no longer alive. We reiterate that the United States government holds the captors fully responsible for the safety of all the hostages."

U.S. Embassy officials here, who saw black and white enlargements of the Polaroid photos, said, "It could be Buckley. We have no information to disprove it, and we cannot certify who it is."

Today's Islamic Jihad statement said, "We shall represent irrefutable proof of Buckley's execution." It added that the group, which took responsibility for Buckley's abduction in Moslem-controlled west Beirut 19 months ago, was ready to put Buckley's corpse at the "disposal of the United Nations and its chief, Javier Perez de Cuellar, on condition that 100 Palestinian prisoners are freed from Israeli jails."

The group offered to deliver Buckley's remains 24 hours ahead of the release of the Palestinians. It warned that the U.S. government was responsible for the fate of five other Americans whom Islamic Jihad says it holds.

The proposal to barter Buckley's body for 100 Palestinian prisoners in Israel represented a shift from Islamic Jihad's standing demand that 17 prisoners in Kuwait, held in the bombings of U.S., French and Kuwaiti facilities in 1983, be released.

Islamic Jihad's latest statement on the fate of Buckley increased concern for the safety of the remaining American hostages, all kidnaped in west Beirut between March 1984 and June of this year.

"We warn America and hold it responsible for what happens to the other hostages," the statement said. "We address the families of the American hostages and say with full regret that the American government and President Reagan will be responsible for any harm to the lives of the hostages we are holding."

The communique rebutted U.S. media reports that Buckley was killed months ago, describing them as "lies."

Islamic Jihad vowed to reply "with an iron fist to any blow to the Moslem people of the world. We will even cut off the hands that try to strike at Moslems. Everybody knows that Islamic Jihad carries out what it says."

It called for the expulsion of Israel from the United Nations and said that in response to its recent condemnation of the Oct. 1 strike against Tunis, Islamic Jihad was ready to turn over Buckley's body to Perez de Cuellar.

"In response to the U.N. condemnation and having obtained permission from the families of the martyrs to dispose of the body of the American spy, we in turn put it at the disposal of the United Nations and its chief, Javier Perez de Cuellar, on condition that 100 Palestinian prisoners are freed from Israeli jails. To show our good intentions, we will deliver the body 24 hours before the date of releasing the prisoners in Israel," the statement said.

The latest Islamic Jihad communique came two days after a published statement by the Islamic Liberation Organization claimed that three Soviet Embassy officials kidnaped on Sept. 30 would not be freed before Syria's intentions in Lebanon became clear.

Al Shiraa, a Lebanese weekly considered close to leftist militia groups in Beirut, reported in its latest issue that the killing of Soviet consular secretary Arkady Katkov was not planned but resulted because he had lost blood from bullet wounds he received when he resisted his abduction.