American jet fighters based on the USS Coral Sea escorted the transport plane carrying Egyptian commandos en route to and from Malta for the assault on a hijacked Egyptair flight that cost 57 lives, according to a western diplomat here.

Three senior American military officers who accompanied the Egyptian commandos on the transport plane were meant to communicate with the U.S. Navy jets if necessary and in case the Libyan Air Force attempted any sort of interception, this source said.

The diplomat said the United States tried to get an advance team of the special U.S. antiterrorist Delta Force from Sigonella, Sicily, into Valletta, Malta, that Sunday, but the Maltese authorities stopped them from landing in an American military helicopter.

The three U.S. officers who arrived with the Egyptians would have been useful in coordinating any activity by the Delta Force on the ground in Malta, the source said.

Diplomats here said the presence of the three Americans also had an important symbolic role for the Egyptians in the operation, for which Cairo sought more extensive and direct U.S. participation than had been reported previously.

The western diplomat said that when the hijacking began, Egypt "invited" assistance from the United States and other countries with highly trained antiterrorist units, inquiring specifically into the whereabouts of the Delta Force.

Egyptian President Hosni Mubarak said last week that the United States, Britain and France had all "offered" to help and to dispatch specially trained units. According to diplomats here, U.S. participation was emphasized at least partly because there were American citizens on board the plane.

In addition, the United States maintains through its Office of Military Cooperation here an extensive liaison with the Egyptian armed forces, which receive about $1.2 billion annually in U.S. military assistance. The senior American officer on board the Egyptian transport has been identified by sources in Washington as Brig. Gen. Robert Wiegand, who commands a staff of about 70.

The office comes under the U.S. ambassador, who has worked closely with the Egyptians in the recent hijacking crises. The embassy consistently has declined to comment on that activity.

When the hijacking occurred, according to the diplomat, the Egyptians said to the United States, "We invite you to join us," and asked "Where's the Delta Force?"

The source said it would have participated in the rescue attempt but it was mostly based in the United States and would have taken too long, according to Egypt's assessments, to get to the scene in time.

Accounts of the Nov. 23 hijacking indicate the hijackers quickly attempted to kill five passengers, and Egyptian officials here said they felt they could not postpone an attempt to rescue those remaining on the plane.

According to the western diplomat, the Egyptians also believed that they had to carry out reconnaissance on the scene and get their units into place during daylight hours on Sunday, Nov. 24. The commando assault took place on Sunday night.

A significant part of the Egyptians' political motive in asking for such U.S. involvement, according to diplomatic sources, was to avoid a repetition of the kind of diplomatic confrontation that developed with the United States in the aftermath of the Achille Lauro cruise ship hijacking in early October.

In that incident the United States refused to sign a letter endorsing Egypt's negotiating posture with the ship hijackers, then intercepted an Egyptian civilian aircraft attempting to deliver them to Palestine Liberation Organization officials in Tunisia.

"I think, looking back on the Achille Lauro," said the western diplomat, that the Egyptians wanted the United States "to be associated with them on this and not blow up relations" if the operation was not successful.

Reports from Malta have suggested that the major cause of deaths aboard Egyptair 648 was smoke inhalation after the Egyptian commandos used explosive charges to enter the plane. But the western diplomat said that while this was a "plausible theory," he had not seen any final judgment on the issue and that assessments are continuing in a number of countries.

While the Egyptian government appears to have been interested in sharing some responsiblity for the operation with the United States, Egypt's leftist opposition has attempted to exploit suspicions that Washington pressured Egypt to take the action it did.

"Some people think the attitude of the United States is quite ambiguous, and they are trying to push Mubarak to do things," said Ingi Rushdy, senior diplomatic correspondent for the semiofficial Al Ahram newspaper.

But the western diplomat said the decision to storm the plane was in keeping with events that have unfolded in recent months.

The Achille Lauro hijacking was described as a "great affront to Egypt's dignity," in the sense that it took place off Egypt's coast during a typical cruise including Egyptian ports, even though it was an Italian liner with no Egyptians aboard.

"Then to have it a hijacking done again with something that had Egypt's name on it, under those circumstances I think they reached the point where they had to move," said the diplomat.

"You can argue about whether the rescue killed 50 or saved 40, but I don't think they the Egyptians are going to negotiate anymore," he concluded.

After the assault on the Egyptair plane in Malta, U.S. officials were quick to express sympathy and understanding of the Egyptian move, despite its grim consequences.

Secretary of State George P. Shultz said on NBC-TV that the way to deal with the terrorists is "to get after them with both barrels" and the State Department issued a formal statement that Sunday declaring, "The United States supports the difficult decision of the governments of Malta and Egypt."