Despite reports of civilian casualties, the Israeli Army allowed Lebanese Christian militia units in the Sabra and Shatila refugee camps of West Beirut to bring in fresh troops and restock their ammunition supplies during the second day of the massacre there, a senior Israeli Army officer said today.

Brig. Gen. Amos Yaron, the commander of all Israeli forces in the Beirut area, said he authorized the resupply and troop rotation operation even after he and a superior, Maj. Gen. Amir Drori, had become uneasy about the militiamen's behavior and initially had ordered a halt to the militia units' activities in the camps.

But Yaron, testifying before the state judicial board of inquiry that is investigating the September massacre of hundreds of Palestinian refugees in the camps, said that by then he presumed that most civilians had fled from the area.

Prime Minister Menachem Begin is scheduled to testify publicly before the inquiry board Monday.

Yaron, who at times was visibly nervous, testified for an hour in public before the three-member commission and for another 90 minutes in private. His public testimony raised anew the question of why the Israelis, alerted as early as the night of Thursday, Sept. 16, to the possibility of heavy civilian casualties, allowed the Christian Phalangist militia units to remain in the Palestinian neighborhoods until the morning of Saturday, Sept. 18.

Yaron is the first Israeli officer to mention publicly the resupply and troop rotation operation, which he said involved the replacement of about 100 Phalangist soldiers with fresh troops on the afternoon of Friday, Sept. 17. Other witnesses, including Defense Minister Ariel Sharon, have emphasized that Israel prevented the Phalangists from bringing reinforcements into the camps, suggesting that this showed an effort to restrain the militia units.

Moreover, Yaron's description of the apparent ease with which the Phalangist troops were replaced conflicts with Sharon's contention that it was a difficult, time-consuming process to get the militia units out of the refugee neighborhoods.

Sharon and others, including Yaron today, have maintained that they had no clear evidence of the massacre until after the Phalangist units left the camps on that Saturday.

Yaron testified in Hebrew. Portions of his testimony were translated into English by the government press office, and a pool of Hebrew-speaking reporters who attended the hearing also provided an English-language version of his testimony.

Yaron, adding new details to what is known about Israeli activities during the massacre, indicated that the Israelis were distrustful of the Phalangists' method of operating and attempted to keep a close watch on their Christian allies.

Before the militia units entered Sabra and Shatila on Sept. 16, Yaron said, he warned them against killing civilians. He said he issued the warning because he knew "their norms are not the same" as those of the Israeli Defense Forces.

As an extra precaution, Yaron said he ordered Israeli soldiers to monitor Phalangist radio communications and to observe the refugee camps from the roofs of nearby buildings. These steps were in addition to the presence at Phalangist headquarters during the massacre of an Israeli civilian, believed to be a member of the country's intelligence service, Mossad, according to earlier testimony by Drori.

Yaron said that on the night of Sept. 16 he received "disorganized reports" that 120, 300 or 45 civilians and Palestinian guerrillas had been killed. These reports, he said, were enough to make him believe "there is likely to occur here something beyond fighting against the terrorists."

Yaron said one report in particular troubled him. This was an interception of a radio communication in which a Phalangist soldier asked an officer what he should do next.

"Do what God commands you," the officer replied, according to Yaron.

Yaron, however, said he did not act on any of these reports or inform his superior officers of them because they were impossible to verify and because of the tendency of soldiers in battle to exaggerate.

"Did you sleep well" that night, Israeli Supreme Court Justice Aharon Barak, a member of the commission, asked Yaron.

"Not bad, " he answered.

Drori, the Israeli Army's northern commander, testified that he ordered a halt to the Phalangist operation in the camps late on the morning of Sept. 17 because of a "bad feeling" conveyed to him by Yaron. Yaron said today his uneasiness was based on fragmentary reports, among them that a woman had been struck by a rifle butt and that a Palestinian child had been kidnaped.

Asked if he had not been told by another Israeli officer earlier on the morning of Sept. 17 that civilians were being killed, he said, "I don't remember, but perhaps."

According to Yaron's testimony, it was sometime after Drori issued the halt order that he authorized the ammunition resupply and troop rotation. Pressed by the commission on why he did this after hearing reports of civilian casualties, he said, "Well, we didn't think a massacre was going on."

He said the Phalangist commanders argued they needed fresh troops because a number of the approximately 130 militiamen who initially entered the camps had been wounded. Despite Drori's order to halt the operation, Yaron said, firing continued in the camps the afternoon of Sept. 17.

One of the key events the commission is investigating is a meeting at 4:30 p.m. Sept. 17 among Phalangist commanders, Yaron, Drori and Lt. Gen. Rafael Eitan, the Israel Army's chief of staff, at which Eitan authorized the militia units to remain in the refugee camps until the next morning.

In defending this decision, Sharon and Drori have suggested in their testimony that the Phalangist commanders had asked for additional time because of the difficulty of extracting their men from the heavily urbanized camps.

Yaron indicated that Eitan had no reluctance to allow the militia units to remain in Sabra and Shatila until the next morning. He testified that the main reason the Phalangist units were ordered out of the camps on Sept. 18 was not fear of civilian deaths but because unnamed American officials were pressing the Israelis to have them removed.

Yaron quoted the Phalangists as saying in response to questions about the situation in the camps: "All is okay. There is just one thing: the Americans want us to leave the camps," United Press International reported.

He acknowledged under questioning that the subject of civilian deaths came up during the meeting. After being assured the deaths were not excessive, he said, Eitan "congratulated" the Phalangist commanders on their performance and agreed to give them one Israeli tractor to "destroy illegal buildings" in the camps. Eitan's testimony before the inquiry board was entirely closed.

Meanwhile, in the occupied Gaza strip, an Arab man was killed today and four others injured when a hand grenade thrown into a passing Israeli Army jeep was tossed back by the driver, the military command announced. It was not clear from the military communique who threw the grenade at the jeep.