The Israeli government charged yesterday that the number of Palestinian terrorists who have infiltrated back into southern Lebanon has grown to more than 400, and that the U.N. peacekeeping forces there still maintain tacit agreements with the terrorists to let them pass.

An Israeli Foreign Ministry spokesman, calling the infiltration rate a "very worrying situation," said that right-wing Christian militia units supported by Israel are unable to prevent the movement of guerrillas because they are too busy defending themselves.

The Palestine Liberation Organization guerrillas, the spokesman said, are moving back south of the Litani River with arms, and that "some of the units of UNIFIL (the U.N. peacekeeping force) by agreement look the other way."

On March 14, following a PLO attack on a tourist bus near Tel Aviv, Israeli land and air forces invaded southern Lebanon to destroy permanent PLO bases south of the Litani. With the deployment of 6,000 U.N. troops along the 64-mile-long border south of the river, the Israeli Army withdrew the last of its forces on JUne 13.

The Foreign Ministry spokesman said that Israel had not made a complaint to the U.N. since Foreign Minister Moshe Dayan wrote a letter last month to U.N. Secretary General Kurt Waldheim accusing UNIFIL of cooperating with the terrorists, letting them pass to the south and permitting food and supplies to reach them.

Waldheim denied the U.N. forces were cooperating with the PLO, but he said that for "humanitarian and practical reasons" the U.N. had an "ad hoc arrangement" allowing the delivery of non-military supplies to Palestinian groups in the control area.

Citing reports from the peacekeeping area, the Jersusalem Post yesterday reported that the agreements have no been made with the U.N. command in southern Lebanon, but with individual units.

A U.N. spokesman here said, "There is no agreement with anyone. They (the PLO) have managed to infiltrate the lines, but wheneber they are found they are turned back."

The spokesman said that the number of infiltrators is not known because it is constantly changing. He said the U.N. has evidence that some terrorists have been returning to north of the Litani because they are unable to move south of the U.N. deployment.

The Foreign Ministry spokesman, when asked how Israeli intillegence has established that 400 terrorists were back in the south, said, "We know it from good sources." Half the PLO forces reportedly are in the eastern sector and half in the west. The central sector is controlled by an Iranain U.N. contingent and is believed to be free of PLO units.

The Foreign Ministry said that most of the agreements with the terrorists had been made with French Peacekeeping units, and that there has been "some problem" with the Norewegains. UNIFIL also has untis from Ireland, Senegal, Nigeria and Fiji.

Last month, PLO leader Yasser Arafat reportedly told Ghanaian Gen Emanuel Erskine, the U.N. Lebanon peacekeeping commandor, that the PLO would "cooperate with UNIFIL and would not initiate hostile acts against Israel from southern Lebanon, although it would continue such acts from other areas."

Since the invasion, there have been numberous bombings and attempted bombings in Israel and one PLO attack on a kibbutz in northern Israel by PLO terrorists who had crossed the Jordan River, but there have been no terrorist attacks from southern Lebanon.