The Rev. Benjamin Weir, freed after 16 months of captivity in Lebanon, said yesterday that his Moslem extremist kidnapers had released him to warn President Reagan that they are ready to kidnap and possibly execute more Americans if there is no response "soon" to their demand for the release of 17 terrorists imprisoned in Kuwait.

"They state that if there is not a positive response to their demand in the near future, they are prepared to kidnap other Americans and that, though they do not want to harm anyone, they will go so far as to proceed to execute their hostages," Weir, speaking in a calm, firm voice, told a nationally televised news conference at the National Presbyterian Church here.

Relatives of some of the six remaining American captives in Lebanon were in the audience; some wept at hearing Weir's somber message.

U.S. officials responded coolly. White House spokesman Edward Djerejian repeated the administration's policy of refusing to "give in to the demands of terrorists" and said it will not pressure Kuwait. President Reagan, he added, watched Weir's televised plea for stepped-up U.S. action with "a great deal of interest and enormous sympathy."

Similarly, the Kuwaiti ambassador here said his government will not change its policy of refusing to negotiate the release of the 17, who were convicted in March 1984 for a series of bombings.

"We see no connection between the convicted terrorists in Kuwait and the American hostages in Lebanon," Ambassador Sheik Saud al-Sabah said.

He also noted that the United States had "repeatedly" associated itself publicly with the tough Kuwaiti policy of refusing to give in to terrorists' demands and said he did not expect any change in the U.S. attitude.

Reagan, in a brief impromptu session with reporters later, said of criticism directed at him by hostage families, "Unfortunately we can't tell even the families all the things we are doing, so we just have to take that criticism, but it is not justified."

The remarks of State Department spokesman Bernard Kalb indicated a desire to avoid appearing callous given the public emotion stirred by Weir's comments.

"We will review the Rev. Weir's suggestions carefully and will continue to do everything possible consistent with U.S. policy to obtain the expeditious release of the remaining six hostages," Kalb said. "This is one of the highest priorities of the administration."

Officials "will continue to be in touch with numerous individuals and governments in the area in an effort to obtain the release of the hostages," he added.

A senior administration official who asked not to be identified said the United States did not negotiate Weir's release. Asked if Syria had played a role, he would only say, "There is a distinction between those in a position to help and those who have helped."

Syria has complained recently that the United States did not live up to a bargain it claims was reached in connection with the release of 39 TWA hostages in June. U.S. officials have denied that there was an agreement for Israel to release 750 Shiite prisoners in turn, but one acknowledged that "we let the Syrians assume" there was.

Syrian officials' resulting anger at the slowness of the Israeli release has led them to withhold their influence inside Lebanon to win freedom for the remaining Americans, according to some officials.

The senior official also said there is a credible basis for believing that all six Americans are still alive, despite speculation that two might be dead.

Weir said he had met with four of the six, in more than one location, in recent months. He last saw the four -- the Rev. Lawrence Martin Jenco, Thomas Sutherland, David Jacobsen and Terry Anderson -- alive and well last Saturday just before his sudden release, he said.

But he has no information on two others, Peter Kilburn, who has a heart ailment, and William Buckley, he said.

"A window of opportunity has been opened at least a crack and the opportunity for negotiations should be seized," he said. "I fear that opportunity might not last long."

Regarding U.S. policy, he said, "I urge that new efforts be made and that new and creative options be explored for negotiating their release."

Flanked by his wife and four children, the mild-mannered missionary looked fit and poised throughout the 50-minute news conference. "To my surprise and deep gratitude to God," he said, he feels in "top physical shape" and doctors here have confirmed it.

He compared himself to Rip Van Winkle, saying he had been held in "complete isolation" until last July 2 and had no knowledge of events such as the June hijacking of TWA Flight 847 and the subsequent release of those hostages by Moslem radicals at about that time.

On that July 2, he said, "it was like Christmas came." After that, he was allowed to meet with the other hostages on occasion, sometimes blindfolded. He also was given an occasional English newspaper and later was allowed to listen to news broadcasts on the radio.

He said he was kept on a "simple Lebanese diet," lacking in fresh fruits and vegetables, but one to which he was accustomed, having lived in the country since 1953. He and Jenco, a Roman Catholic priest, were treated with special respect, he added, because of their religious status.

Jeremy Levin, former Beirut bureau chief for Cable News Network and a hostage for 11 months before escaping last February, was seated with the hostage relatives. He rose to ask Weir whether he felt he had been brainwashed.

Weir answered that because of his ties to the Shiite community, he felt "some measure of understanding and certainly appreciation" for the people. "But I do not believe my attitudes have changed in any decided way. I deeply resent the injustice of having been kidnaped," he added.

He had only 90 minutes warning that he had been chosen for release, he said. He told the Associated Press later that "we all had expected it would be Terry [Anderson]," Beirut bureau chief for AP, who would be released, if anyone was. In the four days before that, his captors seemed to be debating who would do the best job of communicating their demands to the Reagan administration.

Anderson, 37, "was deeply disappointed when he found out he would not go free." Jenco "put his arm around him. It seemed to comfort him."

Asked how he felt about his return to freedom, Weir said, "To me it was a tender time of realizing perhaps I was saying goodbye to Lebanon for the last time."