Israel and Syria declared a cease-fire yesterday, halting major fighting in Lebanon after six days of heavy ground and air combat, but Israeli officials said the cease-fire did not apply to the "terrorists" of the Palestine Liberation Organization.

Minutes before the noon cease-fire went into effect, Israeli bombers destroyed a Beirut apartment building housing the PLO's military command center. A PLO communique said more than 100 persons had been killed, but Beirut radio put the toll at five dead and 82 injured.

Israel said its warplanes shot down 18 more Syrian Migs in morning air battles yesterday, bringing the total of Syrian combat planes lost this week to 79 by Israeli count--nearly one-fifth of Syria's known air strength.

U.S. officials reacted cautiously to the announcement of the cease-fire. Secretary of State Alexander M. Haig Jr. called it "a hopeful sign" but said that "without some kind of disengagement" the situation remained "volatile and dangerous."

Israel declared the cease-fire after the Cabinet and Defense Minister Ariel Sharon said that the massive invasion force had "successfully completed the mission" of clearing lower Lebanon of Palestinian guerrillas to halt attacks on northern Israel. Shortly after the Israeli announcement, Syria ordered its forces in Lebanon to stop firing, and as of late last night, the cease-fire appeared to be holding.

Israel warned that any attempt by Syria to try to move new antiaircraft missiles into Lebanon, to replace those destroyed by Israel in massive bombing raids Wednesday, "will be repulsed without delay and with all the means at the disposal of the Air Force."

Syrian President Hafez Assad said in Damascus that the Syrian cease-fire was ordered with the understanding that Israel would completely withdraw its forces from Lebanon. Israel, in its declaration, made no commitment to do so, however, and Sharon, in a news conference in Tel Aviv, said the invasion force "will remain as long as necessary."

Officials in Damascus said the cease-fire agreement was reached after a meeting Thursday night between Assad and U.S. special envoy Philip C. Habib and that Habib took Syria's "conditions" to Jerusalem yesterday morning.

A senior Israeli official denied that Habib had played a coordinating role in the cease-fire, however, insisting that Israel had acted "unilaterally" and that the declaration was not in response to pressure on Prime Minister Menachem Begin by President Reagan.

Sharon said the next step envisioned by Israel would be to "reach an agreement guaranteeing peace and quiet along Israel's northern border and freedom of action to Israel in its fight against terrorism anywhere else in the world." An Israeli official said a Cabinet committee would draft a proposed settlement along these lines and another official said he foresaw Habib playing a role in these negotiations.

The cease-fire left Israel in control of about 1,100 square miles of Lebanese territory--nearly one-third of the Connecticut-sized country. The area includes nearly all of the Palestinian guerrilla bases and PLO strongholds in Lebanon, except for Beirut, the capital city, as well as most of the 400,000-strong Palestinian community living in Lebanon.

Israeli tanks and troops continued to mass on Beirut's southern outskirts yesterday and heavy artillery exchanges were reported throughout the day, with most of the fighting taking place near the Beirut International Airport. Israeli planes and gunboats bombarded Beirut, but Israeli troops made no move to enter the city and officials in Jerusalem said leaflets dropped on the city yesterday threatening to capture it were merely "psychological warfare."

The Israeli bombing raid just before noon apparently made a direct hit that collapsed a concrete building in which the PLO had its military command headquarters, according to eyewitnesses, who said the death toll appeared to be heavy.

PLO spokesmen said more than 100 people, "most of them civilians," were killed and several guerrillas were trapped in an underground bunker. But they said no high PLO officials were among the casualties.

Both Sharon and PLO officials made it clear that there was no cease-fire between Israel and the Palestinian guerrillas.

"We do not have a cease-fire with the terrorists," Sharon said. "We do not have a dialogue with terrorists whose avowed purpose is the destruction of Israel." He added that the cease-fire also would not "prevent cleaning and purifying" the portion of Lebanon controlled by Israel.

Salah Khalaf, also known as Abu Iyad, a high PLO official, said in a radio broadcast that "we are not concerned by the cease-fire in any way."

Both Syria and Israel acknowledged yesterday that they had suffered heavy losses in the air and tank battles.

Sharon said Israel's casualties so far were "over 100 dead and about 600 soldiers injured--a dear price--including many commanders." Israel announced early yesterday that Maj. Gen. Yekutiel Adam, a former deputy chief of staff, had been killed in a guerrilla ambush near Tyre in southern Lebanon, along with a colonel. Adam had been nominated as the next head of the Mossad, the Israeli intelligence service.

In addition to the 79 planes Israel claims to have shot down and the 19 SA6 missile sites it claims to have destroyed, Syria said yesterday it lost 83 tanks Thursday, while destroying 164 Israeli tanks and armored vehicles. Large tank battles were reported Thursday in the Lake Qaraaoun region of central Lebanon near the Bekaa Valley, the strategic area where Syria had massed most of its 39,000 troops.

Sharon said Syria had lost nine of its T72 tanks, the most advanced made by the Soviet Union, Syria's chief arms supplier, in battles this week with the Merkava, an Israeli-built tank. The Lebanese combat was the first for both tanks.

Israel launched its invasion, its first major military campaign since a smaller but similar invasion of southern Lebanon in 1978, on Sunday, three days after its ambassador to Britain, Shlomo Argov, was critically wounded in an ambush.

Three men--two Jordanians and an Iraqi--were arrested and Israel immediately held the PLO responsible, although an anti-PLO Palestinian faction claimed responsibility.

Within hours of the shooting, Israel began bombing Beirut and southern Lebanon and the raids continued until Sunday night when Israeli troops invaded the country.

With heavy air cover and bombardment from gunships, the Israeli forces moved rapidly northward, knocking out guerrilla bases and capturing PLO-controlled cities, but reportedly leaving pockets of guerrilla resistance untouched.

By the time the cease-fire was imposed yesterday, the Israeli invasion force was estimated at about 40,000--nearly one-fourth the country's regular armed forces.