Special U.S. envoy Morris Draper, in unusually strong language, accused Israel of responsibility for the "terrible" and "obscene" massacre in two Palestinian refugee camps in Beirut in September within minutes of learning of the high death toll there, an Israeli official testified today.

Bruce Kashdan, the Israeli Foreign Ministry's representative in Beirut, said Draper telephoned him the morning of Saturday, Sept. 18, to insist that Israel "stop the massacres" by Lebanese Christian forces. The night before, Kashdan said, Draper had called to warn Israel against allowing the Christian militia into the camps.

The Israeli board of inquiry that is investigating the massacre of more than 300 Palestinians also heard testimony today by Prime Minister Menachem Begin's military aide, Lt. Col. Azriel Nevo, denying that he received any reports regarding the two West Beirut refugee camps on Sept. 17, the second day of the massacre.

This contradicted testimony earlier by an Israeli official who said he had relayed reports of "alleged irregularities" to Nevo that day.

Kashdan's testimony provided the first public details of official U.S. reaction at the time of the massacre and of American attempts to halt it. Later that same day, President Reagan expressed his "outrage and revulsion" and accused Israel of indirect responsibility because of its actions in allowing the Phalangist militia into the virtually undefended camps.

Kashdan said that between 7 o'clock and 8:30 on the night of Friday, Sept. 17, Draper called him with a report that unidentified soldiers had entered Akka Hospital and that Phalangist militia units were in the Shatila refugee camp.

"And he told me, in his words, 'using Phalangists in West Beirut could have horrible results,' " Kashdan said.

Kashdan said he checked this information with the Israeli Army and was told that the Phalangist units had entered the Palestinian neighborhoods through Lebanese Army lines north of where the Israeli Army was stationed. This was one of the Israeli Army's early versions of how the Christian militiamen had entered the camps, although it was later revealed that their movements were fully coordinated with the Israeli Army.

Kashdan said that he relayed all of the information he received from Draper to David Kimche, the director general of the Foreign Ministry. Kimche testified before the investigating panel earlier in closed session.

Kashdan also described Draper's reaction on Saturday, Sept. 18, when the extent of the massacre began to be known, saying the American diplomat told him he had "a message for Mr. Sharon," referring to Defense Minister Ariel Sharon.

" 'You must stop the massacres,' " he quoted Draper as saying. " 'They are obscene. I have an officer in the camp counting bodies. You ought to be ashamed. The situation is rotten and terrible. They are killing children. You are in absolute control of the area, and therefore responsible for that area.' "

Kashdan said he had a later conversation with Robert Pugh, the deputy chief of mission at the U.S. Embassy in Beirut, who told him, "Regardless of what the press has published, we [the Americans] didn't know of the massacre before Saturday. We knew something was happening, but not the extent, until we went in on Saturday."

Begin's aide Nevo, in his testimony, denied talking on Friday to Hanan Bar-On, the deputy director general of the Foreign Ministry. Bar-On, appearing before the inquiry board last week, said that early Friday night he had relayed to Nevo a report of "alleged irregularities" in Beirut from an unnamed American official.

"On Friday, as far as I can remember, I had no conversations with Bar-On," Nevo said. "He says he called me at home, but I was not there."

Throughout his testimony, Nevo maintained that no reports reached him that would cause concern about the refugee camps. He said he last talked to Begin at 2:30 p.m., before the beginning of the Jewish Sabbath on Friday, and that the subject of the camps did not come up because "it had been reported to me that everything was quiet."

From the questioning, it appeared that the inquiry board is still attempting to determine whether Begin heard anything about the events in West Beirut before 5 p.m. Saturday, which is when the prime minister says he first learned of the massacre from a radio news report.