Israeli fighter-bombers, for the third day, bombed and rocketed shattered Palestinian positions and refugee camps today and Lebanese officials expressed growing concern at Israeli armor and troop movements near the port city of Jubayl, nearly 20 miles north of Beirut.

The increased pressure on the Palestinians and Lebanese gave urgency to the mission of U.S. special envoy Philip C. Habib, who arrived here tonight from Jerusalem in an effort to work out a compromise agreement to end the 60-day Israeli siege of Beirut.

Although Lebanese sources who talked with Habib on his return said he was optimistic and that the talks in Israel were showing "good progress," the echoes of high-explosive shells in the city below him were an indication that his efforts to encourage a cease-fire to allow the negotiations to progress were a clear failure.

As Habib returned, Lebanese officials expressed growing concern that Israeli Defense Minister Ariel Sharon, architect of the invasion that began June 6, might be preparing to take his campaign north and east of Beirut to attack other Palestinian and Syrian bastions extending up the Lebanese coast toward the northern port of Tripoli, regardless of whether an agreement is reached to bring peace to Beirut.

The deployment of an Israeli armored brigade to Jubayl, the Phoenician port also known as Byblos, came amid unconfirmed reports of a sea landing of Israeli troops and armor near the port during the night. Israeli tanks also were sighted moving from the coast east of the port to take up positions along the crest of the Metn Mountains that separate the Christian region along the coast from the Bekaa Valley, where 25,000 to 30,000 Syrian troops remain entrenched. Lebanese Prime Minister Shafiq Wazzan met today with President Elias Sarkis to express his concern over these moves.

Western diplomats and Lebanese officials here speculated that the Israeli advances to the north and northeast could be an indication that Israel was contemplating a new campaign against the port of Tripoli, defended by an assortment of Palestinians, Lebanese leftists and Syrian troops, or preparing to outflank the Syrians in the Bekaa.

"The Israelis are not moving north and onto the mountains because they are threatened from there. After all, there is a Christian buffer zone in that area that separates them from the Syrians," said one European diplomat who did not want to be named.

He suggested that Israel "may be planning once again to take a unilateral military action to avoid having to enter negotiations that would involve their own retreat from Lebanon as well as the Syrians'."

The Voice of Israel radio announced today that Israeli tanks and soldiers at Juniyah, midway between Beirut and Jubayl, were stationed there "to prevent any but authorized peace-keeping forces from landing," Washington Post correspondent Leon Dash reported. Israeli military spokesman Col. Yehiel Ben Zvi added that the Israeli government expects any landings of peace-keeping troops "to be coordinated well in advance with" the Israeli Army.

The Voice of Israel also said 40 Israeli tanks had moved into the Lebanese interior east of Jubayl.

In a drive from Beirut to Jubayl today, Dash saw Israeli Army truck convoys moving equipment along the coastal highway a few miles north of Juniyah but none in or near Jubayl. Israeli soldiers were bivouacked on part of Juniyah's northern beaches, Dash reported, and the Israelis also were setting up a large camp in the suburb of Jal el Dib north of Beirut.

Whatever the Israeli intent was in the north, it appeared today that although an agreement to end the violent destruction of West Beirut seemed to be tantalizingly near, Sharon intended to continue pummeling the city and the Palestine Liberation Organization troops inside it until the minute the accord was finally signed.

As they have for the past three days, Israeli fighter-bombers today screamed out of the skies over the capital against ineffective antiaircraft fire to bomb and rocket PLO positions amid what is left of the broken refugee camps of Burj al Barajinah, Sabra and Shatila on the southern outskirts of the city.

The bombing was followed by a steady shelling of a wide swath of southern Beirut, from the international airport north across the seaside embassy district to the once-thriving Corniche Mazraa boulevard, by heavy artillery on the hills around Baabda and from gunboats cruising off the coast.

Heavy fighting also broke out today around the shell-holed National Museum, one of the traditional crossing points between Moslem West and Christian East Beirut.

The Palestinian news agency Wafa said tonight that the fighting began when Israeli forces tried to move down the road from the museum with tanks and an armored bulldozer to attack earthen PLO fortifications guarding that entry into West Beirut. Wafa said that the Israeli advance was repulsed after one tank and the bulldozer were knocked out by the PLO guerrillas.

Israeli radio later confirmed the fighting at the museum and reported that nine Israeli soldiers had been wounded in the battle, which raged most of the afternoon.

Beirut radio, which described today's fighting as "unbelievably violent," said six persons were killed and 17 wounded as a result of the PLO shelling of East Beirut behind the museum area.

In the past, Israeli officials repeatedly have blamed civilian casualties on the fact that the PLO has placed its fighters in civilian areas. But today the Israelis seemed to be using similar tactics, placing tanks and self-propelled 155-mm artillery pieces in still-populated areas of East Beirut, drawing heavy return fire from the PLO that rained down on the hilly warren of apartments and businesses in the Ashrafiyeh district.

One 155-mm gun and an M60 tank fired on the museum area this afternoon from positions just down the street from the Hotel Alexandre, where diplomats and journalists seeking to avoid the violence of West Beirut have sought refuge.

The 155 blew out windows of an adjacent school with the concussion of its shelling and the reverberations shook the hotel, as did the PLO artillery shells that landed in the neighborhood returning fire.

Although the Israeli fire today generally was concentrated on southern Beirut and the crossing points on the Moslem sector's northeast edges, shells also fell in the heart of the city's business district for the first time since last Friday, when Israeli jets demolished a central apartment building that housed a PLO operations room.

One Israeli artillery shell landed on an apartment roof this morning two blocks from the Bristol Hotel, where the International Red Cross is setting up an emergency field hospital in a basement to help take the overflow from Beirut's crowded hospitals.

Another slammed into a building directly across the street from the Hotel Commodore, sending dozens of foreign journalists dining by the hotel's empty swimming pool diving under their tables as bits of concrete and mortar whizzed overhead.