The United States could have demonstrated the awesome power of the atomic bomb high over Tokyo Bay before unleashing it on cities to hasten the end of World War II, says senior nuclear physicist Edward Teller.

A miles-high demonstration blast in the sky visible to Emperor Hirohito and 10 million Japanese was feasible and "might have had the same effect" as the Hiroshima and Nagasaki bombings did in finally prompting Hirohito to sway Japan's divided war council in favor of surrender, he says.Teller, 87, made his comments while reminiscing with other veterans of the "Manhattan Project," World War II's crash effort to develop the nuclear bomb.

The forum marked the 50th anniversary of the first major atomic explosion, the July 16, 1945, "Trinity Test" in New Mexico.

Twenty-one-days after Trinity shook and lit up the desert, the United States bombed Hiroshima Aug. 6. Japan agreed to surrender on Aug. 15, less than a week after history's last atomic bombing, at Nagasaki, Aug. 9.

Teller, a Hungarian who worked in Germany until 1935, immigrated to America after the Nazi takeover and in 1941 became a U.S. citizen. For decades a leading nuclear scientist at the Universities of California and Chicago - where nuclear fission was first achieved - and at the Los Alamos National Laboratory, he is known as the father of the hydrogen bomb.

"I think we shared the opportunity and the duty, which we did not pursue, to find . . . a possibility to demonstrate" the bomb, he said. "Now in retrospect, I have a regret."

Two hundred scientists in Chicago had started a petition recommending a demonstration, Teller recalled. The late J. Robert Oppenheimer, his chief at Los Alamos, opposed taking any stand, Teller said. "I listened to Oppenheimer with a sense of relief . . . that this terrible decision I don't have to make."

"We could have described how to do it with minimum risk and possibility of success," he said.

Teller envisioned parachuting the bomb from an altitude of 10 kilometers, or six miles, primed to explode after the plane was safely away and at a height where the only casualties might have been blindings of people looking directly upward at the blast.

Had the "Manhattan Project" submitted a demonstration option, then President Truman "whose proper choice this was, could have chosen to do nothing, to drop the bomb or to demonstrate it," Teller said.

A Los Alamos and Chicago colleague, Dr. Harold M. Agnew, said dropping the bomb from 10 kilometers would have been "very difficult," and "anything that would have saved one American life at that time, I was all for." Agnew was one of the scientists aboard the Hiroshima mission.

Top Clinton administration officials used the National Academy of Sciences forum and the Trinity Test anniversary to reaffirm the goal of ending nuclear testing.

"We are prepared for the possibility that the United States has already conducted its last nuclear test," White House science adviser John H. Gibbons said.

Energy Secretary Hazel R. O'Leary, whose department oversees America's nuclear arsenal, said the United States "cannot now shrink from our continuing responsibility" to lead the way to a comprehensive test ban treaty.

O'Leary and Gibbons said promise of a global end to testing was essential in persuading a majority of governments to agree last spring to indefinite extension of the Non-Proliferation Treaty. Under the pact, non-nuclear-weapons governments waive the option to develop them.

The administration reaffirmed its current test moratorium after France last month announced plans for one final test round, but some officials are reported recommending reconsideration.