Intensive efforts are under way by the U.N. Middle East peacekeeping forces to bring the governments of Lebanon and Israel to the negotiating table under the auspices of the deactivated Israel-Lebanon Mixed Armistice Commission of 1949, U.N. sources have reported.

The negotiations, if brought about, would be the first time Israel and Lebanon attempted to discuss territorial disputes over southern Lebanon since immediately after the 1978 Israeli invasion north to the Litani River following a terrorist attack near Tel Aviv.

The U.N. effort to reactivate the long-dormant armistice commission comes against a background of increased Israeli Army and Air Force activity in Lebanon, including bombing raids, alleged overflights by pilotless drones, the building of patrol raids across the border into Lebanon and the fencing off of three square miles of Lebanese territory just north of an Israeli border kibbutz.

A U.N. official who has met recently with the Israelis and the Lebanese in an effort to arrange the negotiations stressed that there are no assurances that anything would come of them, because of the Lebanese government's lack of influence in the south and because of Israel's heightened concern about guerrilla activity that originates in southern Lebanon.

But the source said that the alternatives to establishing an Israeli-Lebanese dialogue are increased tension in southern Lebanon and probably more incursions by Israeli armed forces.

One major obstacle to the meeting is Israel's insistence that the Israel-Lebanon Mixed Armistice Commission went out of existence along with the entire 1949 armistice apparatus at the outbreak of the 1967 war. The Israeli position is that when the surrounding Arab nations declared war on the Jewish state in 1967, they abrogated the armistice.

However, the Lebanese government, mindful that it could never enter into direct negotiations with Israel without incurring the wrath of the rest of the Arab world, is insisting that any Israeli-Lebanese contract be held under the guise of the mixed armistice commission.

"It's more than a face-saving matter for the Lebanese. It's the only possible way they could be seen talking with the Israelis. Unless Israel alters its position on the commission, nothing is going to be done," a U.N. source said.

Moreover, U.N. officials noted, the Israeli government does not appear to be anxious to participate in a meeting in which they suspect that the Lebanese would raise a litany of complaints of Israeli violation of Lebanese sovereignty.

The most recent territorial dispute involves a swath of land inside Lebanon -- three square miles, according to U.N. officials -- that Israel fenced off across the border from the Misgav Am, a kibbutz. In April, Palestinian terrorists attacked a children's dormitory in Misgav Am, killing three Israelis, including a 2-year-old child. Five attackers were slain.

The Lebanese government complained to the U.N. Security Council that Israel "annexed" the land, thereby "effectively moving northward the international boundary." The Lebanese ambassador to the United Nations, Ghassan Tueni, also charged that the Israelis had constructed new "patrol roads" in the same area in Lebanon, near the village of Aadeisse, and had made "major topographical changes" by establishing fixed Army positions.

Israeli Foreign Ministry officials were understood to have told U.N. officials here that fencing off the land is a "temporary" security measure to protect Misgav Am. U.N. officials noted the fenced area includes a deep ravine that affords the Israelis a district strategic advantage.

The Israeli government also maintains that the Lebanese-Israeli border is not permanent but is merely a demarcation line drawn by the defunct armistice commission.

Other alleged Israeli violations in Lebanon that could be discussed in negotiations along with Israeli claims of free movements by Palestinian terrorists in U.N.-controlled areas include:

Lebanese complaints of scores of overflights by Israeli drones in recent weeks. U.N. sources say there have been 12 identified drone flights in the last 10 days. Israel denies the charge.

A resumption of bombing raids both north and south of the Litani River by Israeli warplanes, including the U.S.-built F4 Phantom and the A4 Skyhawk.

Continued harassment of U.N. peacekeeping and observer troops in southern Lebanon by the Israeli-supported, Lebanese irregular forces under Maj. Saad Haddad.

Shelling of Palestinian positions by Israeli units that are brought within their range by a network of Israeli-built roads that jut into Lebanon.

U.N. sources said a new concern among the peacekeeping forces -- although there is no evidence it will happen -- is that Israel might use the final weeks of the Carter administration as an opportunity to increase its incursions into Lebanon, on the assumption there would be less U.S. reaction.