Palestine Liberation Organization Chairman Yasser Arafat has announced a PLO alliance with leftist Lebanese forces to carry out attacks against Israeli units in Lebanon.

Ambush and car-bomb attacks against Israelis units have increased from about one every three or four days last month to three or more daily during the past week, costing the lives of five Israeli soldiers.

The PLO role in the attacks has been murky, although Israeli military spokesmen had long blamed the guerrillas. Arafat's public acknowledgment yesterday of PLO participation, however, left unclear whether Palestinians guerrillas already had taken part in the attacks or were just about to engage in them.

Arafat aides, strongly indicating PLO involvement, say the raids are designed to rally disaffected followers in Lebanon and to put Syria on notice that it could pay a price for promoting internal PLO dissension.

A Palestinian intellectual, who declined to be identified, said that he thought the attacks had given the PLO "more substance."

A senior PLO aide here indicated that Arafat, facing restlessness and open rebellion against his leadership, was attempting in part to respond forcefully to charges that he has let the organization drift aimlessly.

The aide also suggested, however, that suspicions of a Syrian role in the PLO split have figured into the new PLO strategy against Israel.

Bitterly accusing Syria of helping to stir opposition, a PLO headquarters' aide sent to a Palestinian refugee camp here, which is a reserve base for PLO fighters in northern Lebanon, talked today of the expanded campaign of attacks.

"It is a signal to the Syrians," he said. "They want to make a small war within our organization, we will make a big war for them," suggesting that heightened tensions caused by the bombings could spill over into general hostilities involving Israel and Syria.

Syrian and Israeli troops waged a mortar duel Monday night in the no-man's-land that stretches across Lebanon's eastern Bekaa Valley, United Press International reported. It said an Israeli Army spokesman in Lebanon denied the battle occurred, but that state-run Beirut radio quoted its correspondent in the Christian enclave of Zahlah as saying the fight was "going strong."

Beirut radio indicated the fighting 25 miles southeast of Beirut was among the fiercest in months between the combatants squared off in the Bekaa since the PLO was driven from Beirut last September.

Palestinians also see the possibility of a war they believe neither the Syrians nor Israelis want as an opportunity for the internally-torn PLO to play again a major role in the Middle East. To settle a renewed outbreak of fighting, the United States would have to rescind its policy of not dealing with the PLO, said one aide confidently, if somewhat unrealistically.

Arafat, who returned to Lebanon today, said in an interview yesterday with the Kuwait newspaper Al Qabas that a "higher Palestinian-Lebanese command" had been formed to coordinate operations against Israel in Lebanon.

"The Palestinians vigorously support Lebanese resistance with all available men, weapons and organizational skill," he said.

Arafat made the remarks at the end of a tour of Arab states seeking support for his leadership of the PLO.

"An all-out war would be in the interest of Arabs," Arafat said. "I have always urged the Arabs to wage war because this is the only way to rectify political scales in the region."

Although top Arafat aides had broadly suggested recently that the PLO was behind the continuing attacks against Israeli troops here, Arafat's comments were the most explicit to date. His remarks still left the PLO role in some doubt, however.

A shadowy group calling itself the Lebanese National Resistance Movement generally calls newspapers here claiming responsibility for the attacks. Arafat indicated that group included the PLO and leftist Lebanese allies, resembling possibly a coalition that existed before the Israeli invasion of Lebanon a year ago smashed the PLO state-within-a-state.

The rebellion against Arafat's leadership has occurred in the Bekaa Valley among officers who have accused Arafat of appointing incompetent officers to command positions and giving up the armed struggle against Israel.

While it remains unclear just how wide the support for the dissidents is, diplomats, PLO leaders and outside observers have all found a deep well of anxiety among Palestinians.

There are reports of restlessness among the Palestinian guerrillas, both those behind Syrian lines in this northern Lebanese port city and those scattered through the Arab world as part of the pact to end the Israeli siege of Beirut last summer.

Arafat's spokesman, Ahmed Abdel Rahman, recently has been sent here and resumed printing of the PLO's publication, which was suspended last September.

The assignment has fueled speculation that Arafat may move his headquarters here from Tunis, although Rahman flatly denied the PLO chairman had any intention of doing that.

Although Syria, pinning down Israel in Lebanon by refusing to withdraw, has repeatedly warned of the possibility of war, PLO fighters and analysts say the Syrian Army in Lebanon has attempted to rein in PLO fighters seeking to conduct hit-and-run raids against Israeli soldiers along the front lines.

"We penetrate through the Syrian lines," said Rahman. "It is no problem." What he did not say, however, was that Syria forced PLO guerrillas to dismantle rocket launchers intended for Israelis on at least two occasions last month because they were positioned close to Syrian front lines.

But the attacks have continued. Today, there were two reported attempts, both near Beirut where Israeli authorities reported guerrillas fired a bazooka at Israeli troops and another when a bomb-laden car exploded near an Israeli position, killing three men inside the car.

In Bern, Switzerland, President Pierre Aubert said Monday he has formally requested that a U.N. conference on the Palestinian issue not be held in Geneva. He cited security problems.