New contingents of U.N. peacekeeping troops are taking up positions in southern Lebanon, but there is growing pessimism about the force's ability to fulfill its mandate.

Palestinian guerrillas continue to infiltrate U.N. lines to regain positions they held before Israel invaded the area in March, Lebanese regular army units are not joining the U.N. forces, and diplomatic considerations seem to be limiting the scope for action of European units, according to Western military sources here.

As a result, morale among the peace-keeping troops has declined and Palestinian guerrillas have strengthened their hand in the area both politically and militarily, the source said.

Palestine Liberation Organization chief Yassar Arafat has said that U.N. troops have no right to interfere with the presence of his guerrillas in southern Lebanon.

Arafat denied recently that his force have agreed to any U.N. cease-fire, and he threatened to carry out more terrorist attacks similar to the bus hijacking near Tel Aviv in March that killed 36 Israelis and precipitated Israel's invasion of Lebanon.

Under these circumstances, Israel is likely to delay its withdrawal from occupied Lebanese territory, military sources said. Although the area has been relatively calm lately, some analysts fear the situation has the makings for serious new clashes.

According to U.N. Security Council Resolution 425, which established the U.N. interim force in Lebanon the peacekeeping troops are charged with "confirming the withdrawal of Israeli forces, restoring international peace and security and assisting the government of Lebanon in ensuring the return of its effective authority in the area." A major problem has been that the struggling government in Beirut has balked at trying to take control in the southern part of the country.

"The U.N. has just about given up on the Lebanese, a Western diplomat said. It has been hoped that at least a few units of the Lebanese army, which is being rebuilt, would be sent to join U.N. forces in the south, but the government has limited itself to sending a few contingents of paramilitary police.

Although some Lebanese military commanders reportedly objected, civilian authorities ruled out the army's participation because they feared that clashes with the Palestinians could wreck the fragile unity that the army has acquired since it's ranks were shattered by the 1975-76 Lebanese civil war.

Another problem is that the French unit, which has been taking the toughest line against guerrila infiltration, seems to have run afoul of France's pro-Arab policies, which dictate against a showdown with the Palestinians. A somewhat humiliating redeployment has put Sengalese troops between the French and Palestinian guerrillas to avoid further friction. Western military sources said French morale has also been hurt by lack of heavy weaponry and restrictions against retaliating for Palestinian-inflicted casualties.

In addition to two Frenchmen killed earlier this month, several have been wounded by Palestinian fire, including the respected commander of the French battalion, Col. Jean-Germain Salvan.

"When you shoot the French commander, you spit on the flag," one military man said.

"The French came with the feeling they were here to win a war, not keep a peace," he said. He added that the tough French marine paratroopers were probably "better troops than required" for the job.

Other U.N. elements are also having their difficulties. "Some units don't speak English or Arabic or French, so they have communication problems," an officer said. Others have complained about a lack of clear instructions and milirary planning.

Nepalese Gurkha soliders pose a logistics problem because of a requirement for goats as part of their rations.

Iranian U.N. troops are 'pretty good," a military sources said, but he added that the unit guarding a key crossing point over the Litani River into the heart of south Lebanon, has been "fairly ineffective. The Palestinians go trudging across the bridge and they just wave at them."

Other U.N. units, and the ones that have taken the most casualties so far, are the Norwegians and the Senegalese. Newly arrived Nigerian troops are now taking up positions as part of a plan to increase U.N. strength to 6,000 men. There now are 4,140 U.N. troops. The additional units are to be provided by Fiji, Iran and Ireland.

But even with 6,000 men it will be difficult to prevent Palestinian forces from returining to former positions, threatening new clashes with Israel or with U.N. forces, military analysts feel.

"There just no way they can stop Palestinian infiltration," one said.

For the Palestinians it is not a matter of infiltrating but of their "presence" at bases which they say are rightfully theirs under the 1969 "Cairo agreement" between Lebanon and the Palestinians.

Some Lebanese political leaders are now calling for the abrogation of the agreement, arguing thatU.N. Resolution 425 renders it null and void.