Kidnaped U.S. diplomat William Buckley surfaced yesterday in a videotape obtained by a London news agency, saying that he and two of the four other Americans believed held captive in Lebanon are "well" but urging the government to "take action for our release quickly."

The tape did not indicate where or by whom the hostages are being held, and officials of Visnews, which showed the tape to reporters, refused to say how it was acquired.

President Reagan expressed cautious relief at the apparent confirmation that the Americans are alive, but he and other officials stuck by administration policy and would not talk about efforts to arrange the captives' release.

"Believe me, this is very much on our minds," Reagan said. "We haven't forgotten they're in captivity, but I don't think it would be productive for us to talk about what we're doing."

Reagan said the 56-second tape provides the first evidence that the five are alive, "if we can take it for granted that that is recent."

Buckley, the former political officer for the U.S. Embassy in Beirut, was shown holding a Jan. 22 copy of the Beirut French-language daily newspaper L'Orient-Le Jour. Positioned in front of a blank wall, he looked pale but well-groomed.

"Today, the 22nd of January 1985, I am well, and my friends Benjamin Weir and Jeremy Levin are also well," Buckley said. "We ask that our government take action for our release quickly."

Buckley was captured at gunpoint in front of witnesses March 16. Levin, Cable News Network's bureau chief in Beirut, disappeared on March 7. Weir, a Presbyterian minister, was captured May 8.

Two other Americans who have disappeared from the streets of Moslem-dominated West Beirut in the past year were not mentioned on the tape. They are Peter Kilburn, a librarian at the American University of Beirut who has been missing since Dec. 3, and the Rev. Lawrence Jenco, the head of the Catholic Relief Services office in Beirut who was kidnaped earlier this month.

Anonymous callers claiming to be members of the Islamic Jihad (Islamic Holy War) have said that the terrorist group is holding all five Americans and have threatened to try them as spies for the Central Intelligence Agency.

According to a State Department official, only two demands have been received in connection with the captives: that all Americans leave Lebanon and that Shiite prisoners in Kuwait be released.

It was not clear what Buckley was referring to when he said "take action," officials said.

A Democratic source connected with the House Foreign Affairs Committee agreed that "not talking about the negotiations is the name of the game" and added that such a policy contributed to the release of an American held in Lebanon in 1983 -- the only case comparable to the current one. In that case, he noted, voice cassettes of the captive -- David Dodge, president of the American University of Beirut -- surfaced during his year of captivity and were "one element that gave a spur to the diplomatic efforts that led to his release."

Relatives of the hostages, including those not mentioned by Buckley, said they took comfort from the tape.

Lucille (Sis) Levin of the District of Columbia, wife of the CNN bureau chief, called it "a very good sign . . . a signal for negotiations." Her daughter said Levin was awakened before dawn yesterday and informed of the tape by NBC, and later she appeared on the network's "Today" show. The network says it has a "nearly exclusive" working agreement with Visnews.

John Jenco of Joliet, Ill., brother of the missing priest, said the family is "a little disappointed that Father Jenco wasn't mentioned" but still found the tape "encouraging." He said the family is assuming that Buckley is unaware of the two most recent captives.