Egypt allowed a nuclear-powered U.S. Navy ship to pass through the Suez Canal for the first time last weekend in what one official called a "breakthrough" for U.S. diplomacy.

The nuclear-powered cruiser USS Arkansas traveled from the Red Sea to the Mediterranean, moving at night because of Egyptian sensitivity on the issue. The Arkansas was shadowing a conventionally-powered Soviet ship that had steamed through the canal the previous day.

Egypt always has barred nuclear-powered ships from passing through the Suez Canal, fearing that an accident might close the canal. The Navy has been frustrated by the policy, which forces one-third of its ships to take the long route around Africa.

Several administration officials confirmed the precedent-setting transit yesterday on condition that they not be identified by name or agency. They said that Egyptian permission in this case does not mean that a new policy has been established, but they said they hope one is forthcoming.

Egypt did not announce the cruiser's passage, and attempts to obtain comment from the Egyptian Embassy here yesterday were unsuccessful. Several U.S. officials, including U.S. Ambassador Nicholas A. Veliotes in a recent cable, warned that public discussion of the transit could embarrass the Egyptians and set back U.S. efforts to win a permanent policy change.

"Right now, we're all very sensitive because they are," one official said. "They've done something nice and we don't want to spoil it."

Officials said they are not sure why Egypt chose this moment to break their longstanding policy. They said one factor may be good will resulting from the recent deployment of U.S. mine-sweeping helicopters to the Red Sea and Gulf of Suez to help Egyptians clear their waters of underwater mines. The origin of those mines has never been conclusively determined.

Opposition to nuclear-powered ships has been strongest in the Suez Canal Authority rather than in the government, officials said. Despite U.S. assurances that such ships are safe, civilian Egyptian authorities have not wanted to allow passage of anything that they believe could endanger a major revenue-producer like the canal.

In addition, officials said there probably has been pressure from the Soviet Union on Egypt not to change its policy. The Soviet navy is currently far less dependent on nuclear power than the U.S. Navy.

The Arkansas had been shadowing the Soviet helicopter carrier Leningrad for some time, like Soviet ships routinely follow U.S. carriers.