The Zairian government said yesterday that thousands of Angolan - backed rebels have again invaded the southwestern province of Shaba and attacked two major towns, including the mining center of Kolwezi.

Announcing the renewed rebel offensive in the copper-rich province, the official Zaire News Agency said that government troops yesterday beat back an insurgent force, said to include Cuban troops, that tried to storm the railway town of Mutshatsha after a two-hour battle. The small town is about 60 miles west of Kolwezi and about 30 miles from the Angolan border. It was captured by rebel invaders last year.

It also said another attack involving 4,000 rebels took place Thursday on Kolwezi itself but did not make altogether clear whether the town was still in government hands. It was presumed here, however, that it was, since its fall would have been a major event known to Western embassies by now.

The news agency account said the force attacking Mutshatsha included "whites who have been identified as Cubans." During the 1977 fighting Zaire broke relations with Havana, charging that Cubans were leading the rebel invaders. Cuban soldiers assisted the present Angolan government to defeat two rival nationalist groups in the 1976 civil war that followed Portugal's withdrawal from its former colony.

Initial reports on the fighting were extremely sketchy and there was some initial doubt as to their authenticity. The Belgian Foreign Ministry in Brussels said, however, that it appeared the situation was at least as serious as last year's uprising in Shaba Province.

Observers here said that if the Zaire News Agency reports were correct they indicated the fighting was far more serious than before for the government of President Mobutu Sese Seko.

[There are about 80 Americans living in the area that is the scene of the current fighting, a State Department spokeswoman said yesterday. She said they were either missonary families or connected with private American companies operating in the area. Their situation is unknown, she said.]

In last year's 80-day war against Angolan-supported insurgents, the Zairian army prevailed with the help of 1,500 Moroccan troops flown in on French transport planes. To Mobutu's deep disappointment, the United States provided only limited military assistance.

The rebels, which the Zairian leader claimed were armed and trained by the Soviet Union as well as Cuba, failed to get as far as Kolwezi before they were driven back across the Angolan border. After taking Mutshatsha, however, they advanced to within about 30 miles of Kolwezi before dispersing into the countryside when confronted by the Moroccan troops.

There was remarkably little direct combat between the Zairian army and the rebles last year, making the war a shadowy one whose psychological impact was far greater than its real military significance.

The seizure of Shaba Province, however, would probably deliver a mortal blow to the already troubled Mobutu government. it has been faced recently with minor rebellions in several provinces and an alleged coup attempt.

Observers here were struck by the initial boldness of the rebels this time in attacking Mutshatsha, which has just been reinforced by a battalion of government commandos.

In addition, the attack on Kolwezi would seem to indicate that the rebels are far better organized and more agressive than they were last year when their unexpectedly rapid initial advance outstripped their ability to maintain the necessary supply lines.%TThere have been periodic reports out of Zaire over the last two months of a rebel buildup around the Zairian border town of Luashi. As many as 3,000 insurgents were said to be assemblying and making occasional strikes on Zairian military convoys in the region.

This seemed to explain why Mobutu sent a battalion of commandos to the province in early February to bolster the North Korean-trained division stationed there since last year.

The insurgents are thought to belong to the National Liberation Front of the Congo-Kinshaha led by Gen. Nathaniel Mbumba. Its ideology and political orientation remain unclear despite its alleged Cuban, Soviet and Angolan backing.

The Front's army was initially said to have been formed by former Katanga gendarmes who fled to Angola when the secessionist movement under the late Moise TshomPe collapsed in 1963. Katanga was the former name of Shaba Province.

The Katangese Policemen late helped the Soviet-and Cuban-backed Popular Movement for the Liberation of Angola to overwhelm its pro-Western opponents during the 1975-76 civil war in Angola.

It is now through, however, that most of these gendarmes are too old to fight and that the Front draws most of its recruits from dissident Baluba and Lunda tribesmen in Zaire. After last year's fighting, some 240,000 refugees fled Shaba Province into Angola, according to the Angolan government.

The National Liberation Front is one of half a dozen opposition groups now seeking to overthrow Mobutu, who has ruled the vast mineral rich central African country of 25 million people for the past 12 years. So far, there has been little cooperation or coordination of activities among the opposition groups.

In March, the Mobutu government executed 13 men following a "treason trial" involving a total of 91 persons. Among them were 67 military officers. One of those said to be implicated in the conspiracy was Mobutu's own internal security chief, Maj. Panumbule.

Zaire, with a $3 billion foreign debt, is presently facing a major economic crisis that has served to erode Mobutu's popularity and to incite unrest among his people. Inflation is put at 75 percent a year and the shortage of key food commodities has been common.

There have been several bloody local rebellions recently in other provinces. They were suppressed with considerable bloodshed by the army.

The insurgency in Shaba, however, is of an entirely different order because of the probable involvement of Angola in helping to arm and train the rebels. If the Zaire New Agency reports are correct, it is likely the Mobutu government now faces a far more serious challenge that, if not contained quickly, could lead to its downfall.