The FBI has opened an investigation to determine whether a senior U.S. government official has been passing highly sensitive information to the Israeli government, according to sources with direct knowledge of the inquiry.

The investigation was launched in January after the National Security Agency intercepted a secure communication between a senior Israeli intelligence officer in Washington and a superior in Tel Aviv that referred to someone code-named "Mega," and an attempt to obtain a sensitive American document, U.S. government officials said.

The officials said that the context of the conversation led them to believe that Mega may be someone in the U.S. government who has provided information to the Israelis in the past.

The sources, who spoke on condition of anonymity, said the investigation was focusing on U.S. officials who would have had access to the document in question, a secret letter of assurances then-Secretary of State Warren Christopher had given to Palestinian leader Yasser Arafat after successful negotiations to withdraw Israeli troops from most of the West Bank city of Hebron.

In the intercepted conversation, according to two U.S. officials, the Israeli intelligence officer informed his superior in Tel Aviv that Israeli Ambassador Eliahu Ben Elissar had asked him whether he could obtain a copy of the letter given to Arafat by Christopher on Jan. 16, the day after the Hebron accord was signed by Arafat and Israeli Prime Minister Binyamin Netanyahu.

According to a source who viewed a copy of the NSA transcript of the conversation, the intelligence officer, speaking in Hebrew, said, "The ambassador wants me to go to Mega to get a copy of this letter." The source said the supervisor in Tel Aviv rejected the request, saying, "This is not something we use Mega for."

Asked yesterday about the investigation, a senior National Security Council official declined to comment. White House spokesman Michael McCurry, traveling with President Clinton in Mexico, also declined to comment.

Ambassador Ben Elissar said emphatically last night that his government had not engaged in any improper activity in Washington, denying that he had made such a request or that Israel has a spy inside the U.S. government. "I deny it as strongly as one can deny anything," Ben Elissar said in a telephone interview. "I simply think that this allegation is ridiculous. . . . Of course, I cannot guarantee for everybody, but if anybody in this embassy had this conversation or such a thought, he can be considered an utter fool, a fool."

The ambassador added that the allegations must have come from "an evil-intentioned source."

A statement issued by Netanyahu's office in Jerusalem last night in response to questions said: "The reported story is absolutely baseless."

U.S. intelligence officials said that NSA transcripts involving possible counterintelligence matters generally are distributed to officials at the CIA's counterintelligence center, the FBI's Division 5 -- which conducts foreign counterintelligence investigations -- and the State Department's bureaus of diplomatic security and intelligence and research. The intelligence officials said the NSA had done nothing improper.

Nevertheless, said a senior U.S. official who said he had been briefed on the matter, the upper echelons of the FBI were dismayed that the information had been disseminated so widely.

Because of the sensitive nature of the communication, said the source who viewed the transcript, it was recalled by the NSA about 12 hours after it was distributed. "It was taken off the street," he said.

The source said that the transcript of the conversation between the two officers of Mossad, the Israeli intelligence service, was translated from Hebrew to English "awkwardly."

He added that the conversation was not confrontational but took "the form of a collegial debate between two colleagues."

The Hebron agreement, initialed by Netanyahu and Arafat at a border post between Israel and the Gaza Strip, marked a historical turning point in the tortuous efforts to get Israel's governing Likud Party to transfer land and limited governing power to Arafat's Palestinian National Authority. The agreement, which was bitterly opposed by some in Netanyahu's cabinet, required Israel to turn over four-fifths of Hebron immediately and substantial chunks of the West Bank still occupied by Israel by the middle of next year.

A day after the accord was signed, on Jan. 16, Christopher gave separate "side letters" of assurance to Arafat and Netanyahu. Israeli officials disclosed the contents of Christopher's letter to Netanyahu, but the contents of the letter to Arafat have not been revealed.

The most difficult issue in the Israeli-Palestinian negotiations involved the three-stage rollback of Israeli troops from rural sections of the West Bank. Assurances about the rollback do not appear in the accord itself but in the letters from Christopher. Dennis Ross, Clinton's special envoy to the Middle East, said at the time the agreement was signed that the letters were not "guarantees as such" but that they represented an expression of U.S. "willingness to act in accordance with our views."

According to the Israeli newspaper Ha'aretz, the wording of Christopher's letter to Arafat "goes beyond a general moral commitment. It can be said that if Israel deliberately violates the agreement, the Palestinians will be able to turn to Washington with this promissory note."

One official with knowledge of the FBI investigation into the identity of Mega cautioned that much remained unknown. But the official said that if it turned out that a senior U.S. official was passing sensitive information to Israeli authorities, it could prove more serious than the espionage case involving Jonathan Jay Pollard, a former Navy analyst who was convicted in 1986 of selling U.S. military intelligence documents to Israel.

Pollard was recruited to work for Israel by Rafael Eitan, an accomplished intelligence officer who worked for the defense ministry and the prime minister's office. Eitan's Lekem intelligence unit ran Pollard as an agent, paying him to make sure he did not quit the operation.

After his espionage activities were disclosed, Israeli authorities insisted that Pollard was motivated primarily by his love of Israel. They said it had been a "rogue" operation by Eitan, unauthorized by the Israeli government.

To help ensure that U.S.-Israeli relations were not greatly harmed by the Pollard case, Israel's intelligence services, including Mossad, worked with U.S. law enforcement and intelligence agencies to determine what information he had passed on. It included, according to U.S. officials, satellite photographs and information on Arab military systems.

Despite that cooperation, however, many of the documents Pollard delivered to his handlers were never returned to the United States by the Israeli authorities.

Pollard was sentenced to a life term in prison. Although Israeli officials frequently have urged clemency, Clinton rejected such a plea from Pollard last year, citing the enormity of his crime, his lack of remorse and the damage he had caused U.S. security. CAPTION: Jonathan Jay Pollard was convicted of selling U.S. military data to Israel.