A senior Israeli official said today that on Sept. 17, the second night of the massacre of Palestinian refugees in West Beirut, he relayed a report from an unnamed U.S. official about "alleged irregularities" in Beirut to Prime Minister Menachem Begin's military aide.

But the official, Hanan Bar-On, the deputy director general of the Israeli Foreign Ministry, told the judicial board of inquiry investigating the massacre that he did not recall the American official referring specifically to "irregular behavior" by the Lebanese Christian Phalangist militiamen who were in the camps, and does not know whether the report was conveyed to Begin.

Begin has maintained that he knew nothing of the massacre until late on the afternoon of Sept. 18.

Another witness who appeared before the panel today, Maj. Saad Haddad, the renegade Lebanese Army major who commands an Israeli-supplied Christian militia in southern Lebanon, reiterated that his men played no role in the slaughter in the Sabra and Shatila refugee camps.

Haddad said his militia units had kept an agreement with the Israeli Army that they would not cross the Awali River, located just north of the Lebanese city of Sidon.

Bar-On said that at about 6:30 p.m. Sept. 17 he received a call from the American official dealing mostly with reports that Israeli soldiers had entered Banks Street in West Beirut, were using loudspeakers and arresting people.

"He also mentioned rumors and expressed reservations about the matter, emphasizing that these are rumors, that he had received all this information from Washington, to the effect that [Israeli] soldiers were seen entering hospitals -- he did not say which hospitals -- and had allegedly arrested patients," Bar-On said.

"He immediately said then that it is difficult to believe these reports and expressed reservations about them. At the same time, as I remember and according to the existing record, he also said something [about] rumors about the entry of Phalangists into the camps without giving details of which camps."

Bar-On said he relayed this information to Begin's military aide, Lt. Col. Azriel Nevo. He said it was Nevo's job to transmit such information to Begin, although the Army officer was known sometimes to consult with the Defense Ministry before doing so.

Begin has denied testimony by others, including Defense Minister Ariel Sharon, that on the morning of Sept. 18 he inquired of the Army about a report concerning Gaza Hospital, which is located in the Sabra refugee camp.

By Sept. 17, the entire top echelon of the Israeli government knew of the Phalangist entry into the refugee camps, having been informed of it the previous night at a Cabinet meeting. All of the senior officials, however, have maintained they had no reports of a massacre taking place in the camps.

An English summary of Bar-On's testimony was released late today by the government press office.

Haddad, dressed in his Israel-supplied uniform, testified in English. He has denied vehemently that his men played any role in the massacre. A number of survivors said Haddad's men participated in the killing.