The Israeli Army reinforced its "defensive" positions inside Lebanon today and bulldozers began cutting roads linking their fortified troop emplacements, according to officials of the United Nations peacekeeping force.

U.N. officials in Nakura, just across the Lebanese border from here, said 300 to 350 Israeli troops were in southern Lebanon, encamped at three sites three miles inside the border. They showed no signs of preparing to leave, the United Nations said. The troops went in early yesterday in an attempt to plug gaps through which, the government said, Palestinian terrorists have passed on their way to attacks inside Israel.

[At the United Nations, a spokesman for Secretary General Kurt Waldheim had said Israeli troops appeared to have begun withdrawing, but later revised this to say the units had moved about in the area. In Beirut, the Lebanese foreign minister said he will ask Waldheim to consider calling a Security Council session on the issue.]

The Israeli incursion was in response to the Palestinian attack Monday on the kibbutz Misgav Am in which three Israelis, including a 2-year-old child, were killed. All five guerrillas were slain.

U.N. officials at Nakura said the Israeli bulldozers have constructed defensive positions surrounded by barbed wire and have erected tents.

In Washington, the State Department voiced concern over the movement of Israeli troops into southern Lebanon, saying it could complicate peacekeeping efforts and raise tensions in the region.

We are not clear what the ultimate intentions of Israelis are in this incursion and we are continuing our discussions," State Department spokesman David Passage said.

["We understand the deep sense of shock and concern in Israel in the aftermath of the terrible tragedy of kibbutz Misgav Am," Passage added. He would not divulge what the United States had told Israel or comment on consultations with other Middle Eastern governments.]

Army officials continued to refuse to comment on the incursion, the first since last May. The Army also refused to permit reporters gathered at this border town to cross in Lebanon.

The troops were reported to be positioned in the villages of Markabe, with 16 tanks and 12 jeeps; in Shaqra, with 40 soldiers and armored vehicles, and Kunin, with 80 soldiers and eight armored vehicles.

Timur Goksel, spokesman for the U.N. peacekeeping force in Nakura, said the Israeli units in Kunin were in an area that is supposed to be controlled by the United Nations.

Maj. Saad Haddad -- whose Israeli-supported Christian militia control a five-mile-wide, 60-mile-long belt stretching from the Mediterranean to the foothills of Mount Hermon -- said all of the Israeli troops were in the Christian enclave.

Haddad said Palestine Liberation Organization guerrillas were beginning to concentrate just north of the Christian salient.

Meanwhile, U.S. Charge d'Affaires Jim Brown met today with Israeli Defense Minister Ezer Weizman to "seek clarifications" about the Israeli incursion.

The U.N. peacekeeping force in Lebanon issued a strong protest to the Israeli Army, and Syrian forces in Lebanon were reported to be on full alert. The Syrians, who dominate an Arab League peacekeeping force, have been in Lebanon since the end of the 1975-76 civil war. [Reuter reported from Beirut that Israeli combat planes flew low over the southern Lebanese port of Sidon and the Palestinian news agency said Israeli artilery shelled the port.]

Israeli Gen. Avigdor Ben-Gal, Army commander for northern Israel, said in an interview published in the Hebrew newspaper Haaretz that Israel should conduct "offensive actions" against the guerrillas but Israel had not done so for more than six months because of international "political considerations."

Washington Post correspondent Edward Cody reported from Beirut:

Israel's occupation of the Lebanese border hills has set off a wave of anger and resentment among U.N. officials and diplomats from countries whose troops patrol the explosive frontier as part of the U.N. force.

Much of the criticism is aimed at the United States as well as Israel because, in their view, Washington has been too timid in trying to restrain Israel for fear of complicating the autonomy talks among Egypt, Israel and the United States.

"The next thing you know, they'll be coming up here to Beirut and directing traffic," said an Irish diplomat after a tour of the border region.

Washington is seen here as the only effective channel for influencing Israel.