An invasion force is approaching one of the most important centers of the copper belt of Zaire, the basic prop of this country's economy, according to well-informed sources here in the capital.

The invaders, believed to be rebellious Katangans who crossed from neighboring Angola 10 days ago, have captured four towns and are now only 50 miles from Kolwezi, a strategic copper-mining city, the sources said. The force is estimated to number about 1,500.

Katanga Province, now called Shaba, provides 60 percent of Zaire's exports and 75 pewr cent of its foreign-exchange earnings. The government of President Mobutu Sese Seko is already in grave economic trouble, and probably could not survive loss of the province's mining income.

The invasion is considered by observers here to be the most dangerous attack since katanga tried to secede from Zaire, then known as the Belgian Congo, in 1960. This time, however, the Katangans appear to be well armed, well trained, highly organized and benefitting from the full support of the Angolans.

After the 1960 secession attempt failed, about 6,000 Katangans fled across the border into Angola. During the Angolan civil war in 1975 and 1976, the Katangans fought alongside the victorious Popular Movement for the Liberation of Angola, with the understanding that the backing would be reciprocated if the Katangans attempted to take Shaba.

Foreign observers here feel that the Angolans are now providing the needed bases and arms for the Katangans, who were among the best troops in Angola's civil war.

The attack began March 8 at Kapanga, Dilolo and Kisenge, all less than 80 miles from the Angolan border. Military sources said the three-pronged attacks were simultaneous, followed within hours by a communique from the rebel movement's office in Paris claiming responsibility for the attack. A spokesman said the aim was to liberate Zaire and to overthrow the Mobutu government.

Well-informed sources said the Katangans easily defeated the small patrols of the Zaire army forces. Zaire now has six battalions in the area and is sending three more. There are indications that Zaire's casualties have been heavy. With the most serious losses sulfered when Zaire troops crosed a mined bridge on the Lukashi River.

The Katangans subsequently took Sandoa, a town between Kapanga and Kisenge.

It is believed here that the Katangan's real aim is to move down the paved road alongside the Benguela railway, which cuts through the heart of the copper belt, and that the order towns attached are diversionary fronts.

It appears that the Katangans hope to take Shaba Province or at least frighten foreign employees at the mining company into fleeing. Several companies have already begun evacuating workers seriously threatening mining operations.

The seriousness of the threat may be best reflected by the reaction of the U.S. and Belgian governments, who have already begun flying in emergency military supplies.

The first of two planeloads of U.S. supplies, totaling between $1 million and $2 million, was flown in last night. It consisted of medicine, uniforms, fuel tanks and rations - but no weapons. The second plane is expected to arrive Friday. The Belgians are reportedly flying in small arms and mortars.

The March 8 attacks followed a month-long buildup in hostilities between Angola and Zaire, despite Zaire's attempts to normalize relations between the two countries. During the civil war Zaire served as a base for the two liberation movements that opposed the MPLA.

In late February, the Angolan government charged that Zaire was behind an "imperialist plot" code-named "Cobra 77" and supported by the United States to overthrow the MPLA government. The Angolans said a Col. Mike Thompson is head of the operation. Both the Zaire and U.S. governments denied any knowledge of such an operation.

A week later Angola charged that an invading force from Zaire had attacked Panagala, a town in northern Angola, killing 43 villagers.

Zaire deined the allegations.

Since the end of Angola's civil war early last year, the two governments have alternately charged each other with invasion and tried to normalize relations.

Besides the current attack Zaire is showing increasing concern about a border town across the Congo River from Zaire. The buildup reportedly began about three week ago.

From Santo Antonio, the Angolans and their Cuban backup forces appear to be sending their Soviet-supplied arms to towns all along the border.

There are no indications why the Angolans are building up posts along the border. It could be to quell continuing internal guerrilla activity, but Zaire fears a new attack into areas near this capital.

Reports from Shaba are sketchy, but there are indications that the Katangans are armed with 75 mm. recoilless rifles and long-range artillery, any may have a 122 mm. multiple rocket-launcher, the most effective weapon in the Angolan civil war.

The Katangans appear to be methodical in their strategy, taking a town and then extending supply lines before moving on, when the operation could be "a fast and easy walkover" according to a well-informed source here.

Zaire has had problems with its supply lines, since the capital and the trouble spots in Shaba are about as far apart as New York and St. Louis.