Less than three hours before its first scheduled launch since the Jan. 28 shuttle disaster, the National Aeronautics and Space Administration scrubbed the liftoff of an unmanned Delta rocket today for at least 48 hours after engineers found a small leak of rocket fuel.

The postponement reflected a new, extra-cautious attitude among NASA officials in light of the space agency's recent problems.

The three-stage Delta rocket is scheduled to put in orbit a $57.5 million weather satellite that the National Oceanic and Atmospheric Administration will use to detect hurricanes and other storms brewing in the Atlantic Ocean.

But late this afternoon, agency officials announced that engineers had found that about 40 cubic centimeters of a kerosene-type fuel used in the first stage had leaked from its tank, past a valve and into the engine system. The leak was described as minor.

Two subsequent checks failed to find any additional leakage, but the officials concluded that the fuel lines should be flushed and the valve rechecked.

"We believe we probably would have had a safe flight," spokesman George Diller said. "There was no chance of an explosion, but you could have lost thrust in the rocket if the fuel lines were damaged . . . . There was enough concern to put it off."

The launch was tentatively rescheduled for Saturday evening to give the crew an extra day's rest. But if the fuel valve must be replaced, the mission could be put off about 10 days, Diller said.

Although Diller and other officials said the postponement was routine, NASA officials have acknowledged taking unusual precautions to ensure the safety of this flight, following two major setbacks to the space program -- the Challenger disaster and the explosion of an Air Force Titan 34D rocket being launched from Vandenberg Air Force Base in California April 18.

Those extra precautions included a full-scale review of the Delta's solid rocket boosters, an extra preflight conference in which officials were required to sign certificates of flight readiness, and a special technical review conference conducted by top NASA officials in Washington.

Preflight tensions led to yet another clash between NASA officials and reporters here.

News organizations that wanted to set up remote cameras near the launch pad were asked to agree in writing to turn over their film to NASA in the event of "a contingency requiring investigation."

"We definitely want the first use of that film for any investigatory board if we did have a contingency," said Ed Harrison, the director of audiovisual operations at the Kennedy Space Center here.

But the news organizations -- including The Associated Press, United Press International, The New York Times and the Orlando Sentinel -- refused to sign. As a result, the only news photographs of the launch will be taken from about 1 1/2 miles away.

"I guess you all have some amendment in the Constitution about the press doing what it wants," Harrison told reporters.

"No one has any desire to interfere with a NASA accident investigation," UPI photographer Peter Cosgrove said. " . . . They can have our film forever, but we should have first use of it."

The launch of the GOES-7 weather satellite (Geostationary Operational Environmental Satellite) has attracted attention because of the renewed importance of unmanned rockets to the space program.

NASA had planned to phase out its rockets in favor of the manned shuttle with the launches of its last Atlas Centaur rocket next April and its last Deltas in August 1987.

But with the shuttle program on hold for at least a year, there has been pressure to return to unmanned rockets.

In addition, officials of McDonnell Douglas Corp., which manufactures the Delta, have been lobbying NASA and the Air Force to buy new rockets.