A caller claiming to speak for Islamic Jihad, a group holding six Americans hostage here, said today that some of the prisoners would hold a press conference and address a message to the American public and the U.S. government.

The anonymous caller to two news agencies here, did not indicate when or how the press conference would take place or who among the hostages would participate. But he warned that after it takes place, the U.S. government would have to "assume full responsibility for the lives of the hostages" and said that if the United States did not respond satisfactorily to earlier unspecified demands, "our attitude will be decisive."

If the captives, some of whom have been held for more than a year, are produced at a press conference, it would be their first public appearance since they were kidnaped.

While the caller made no demands on behalf of the captors, the warning suggested that demands might be made at the press conference. The Rev. Benjamin Weir, a Presbyterian minister freed Sept. 14 after 16 months in captivity, said Islamic Jihad was prepared to execute the remaining American hostages if Kuwait did not free 17 terrorists it has convicted in a series of 1983 bombings.

U.S. officials said in Washington that they had no confirmation of the reported press conference. Secretary of State George P. Shultz, appearing on NBC's "Meet the Press," said, "We welcome any release, but want all of the hostages back."

Shultz said, however, that the United States would not press Kuwait to free its prisoners. "We don't think it is wise to pressure for the release of people who are being held for . . . killing people there in exchange for the hostages," Shultz said. "We don't agree with the approach of bargaining with people who are kidnaping or hijacking or whatever."

In today's messages, a male, Arabic-speaking caller claimed to represent Islamic Jihad, or Islamic Holy War, a terrorist group that has claimed responsibility in similar calls for a number of kidnapings and bomb attacks against U.S. Embassy buildings and other western targets.

There was no way to authenticate the calls.

The caller read a long statement saying that Islamic Jihad was hostile to American policy but not to the American people.

He said, without elaborating, that the hostages would address the American people and government, their families and humanitarian agencies. "We also confirm to the families of the hostages, American public opinion and world public opinion that as regards what happens after this period, the American government will assume full responsibility for the lives of the hostages," he said.

The six Americans, seized between March 1984 and last June, are William Buckley, political officer at the U.S. Embassy; the Rev. Lawrence Jenco, Beirut director of the U.S.-based Catholic Relief Services; Terry Anderson, Middle East bureau chief for The Associated Press, and three employes of the American University of Beirut -- Peter Kilburn, a librarian; Thomas Sutherland, dean of agriculture, and David Jacobsen, administrative director of its hospital. Several others westerners have been also kidnaped.

Today's caller said Weir had been released to show that "we are not enemies of the American people but of the American policy that is criminal against the oppressed people."

He also said Weir had been freed "to prove our good intentions by releasing hostages and fulfilling pledges we have made and to give the American government an opportunity, which has been extended. We are waiting for it to expire after which our attitude will be decisive."

This was in line with Weir's remark in a press conference in Washington a week ago that "they have released me as a sign of their good intentions. However, they are not willing to wait much longer."

Weir had cautioned that there remained only a small "window of opportunity" for gaining the release of the six remaining Americans. From conversations, Weir said he understood his captors were "taking extreme, radical action to recover the 17 prisoners from Kuwait."

Kuwaiti courts have convicted the 17, most of them Shiite Moslem militants and two of them Lebanese, in bombings at the U.S. and French embassies and other sites in Kuwait on Dec. 12, 1983, in which five persons were killed and 86 wounded.

Well-placed Arab diplomatic sources and relatives of the American hostages have said that they understand Islamic Jihad would settle for the release of the two Lebanese prisoners in Kuwait.

Some observers here said it appeared that the kidnapers were becoming desperate for some movement on the issue of the prisoners being held by Kuwait. Kuwaiti leaders have said they will not be pressured but press reports two months ago said Kuwait was seeking a pledge that no terrorist action would be taken against it if the bombers were freed.

There was no indication from the caller about how a press conference might be held. There was speculation that because a live conference could disclose the whereabouts of the hostages or the identities of their captors, the kidnapers might conduct a videotaped conference privately, as they have in the past.