Heavy fighting appeared to be continuing yesterday around the main mining center of Kolwezi in Zaire's copper-rich southern province of Shaba and some reports said rebel forces had seized control of the town and rounded up hundreds of foreigners, including about 75 Americans, living there.

Accounts of the situation available here and in European capitals remained fragmentary and often contradictory, however. Some indicated the Zairian army had recaptured several strategic points in Kolwezi, including military headquarters, but others spoke of the fall of both the mining center and the railroad town of Mutshatsha, 60 miles to the west, to well-armed rebels of the Congo National Liberation Front.

It appeared that the Zairian army's initial air and ground counteroffensive had failed to dislodge the Mutshatsha, despite the use of French Mirage jets to bomb and strafe rebel positions.

Observers here felt the situation was far more serious for the Zairian government than last year when the same rebel group tried to topple President Mobutu Sese Seko by seizing Shaba Province but failed after France and Morocco came to his rescue. Shaba is the former Katanga Province, known for its rebelliousness against the central Zairian government.

The rebels were reported to be gathering more forces to attack other main towns along the road between Kolwezi and the Angolan border, possibly to consolidate their road supply lines into central Shaba. The total number of insurgents is unknown but the Zairian government said Sunday that 4,000 were involved in the attack on Kolwezi alone when it began last Thursday.

Zaire has charged that the rebels entered from Angola and are being supported by the Marxist government in that former Portuguese colony. Zaire backed the opponents of Angola's present rulers in the civil war that followed independence there in 1975 and is reported to have continued assisting an Angolan faction still in rebellion.

[In Washington, State Department spokesman Hodding Carter said, "We are, as last year, concerned about the territorial integrity of Zaire . . . and are] opposed to foreign intervention there.

[Under the "longstanding relationship" between the United States and Zaire, Carter said, in the current fiscal year the United States is supplying $48.5 million in military and economic aid and credits to Zaire. The State Department had established a task force to keep abreast of the latest military incursion into Zaire, but the United States has not yet determinded "a definitive course of action."]

Reports reaching here said at least half a dozen Europeans had been captured by the dissidents in and around Kolwezi. All but one of the Americans working for the big Inga-Shaba ConstructionCo. base there and their dependents were reported safe and accounted for.

About 3,000 Belgians, French and Americans are in the Shaba region affected by the fighting. Some were reportedly evacuated yesterday from another mining site at Fungurume about 60 miles east of Kolwezi on the road and rail line to the Shaba capital of Lubumbashi.

It was not immediately clear whether the rebels, whose announced goal is the overthrow of President Mobutu's 12-year-old government were actually holding the Europeans in Kolwezi as prisoners and possible hostages or simply seeking to protect them from the ongoing fighting. One report said there had been no sign of hostility shown toward any whites by the rebels.

If it emerges that the Europeans are being held as hostages, analysts here feel the whole character and international implications of the Shaba war would radically change and possibly provoke the direct intervention of the Western powers to rescue them. This happened in 1964 when American aircraft dropped Belgian paratroopers on Kisangani, then called Stanleyville, to save the lives of Europeans being held hostage by leftist rebels there.

The Mobutu government has already launched an appeal to the West for help, claiming that the rebels are an invading force from Angola trained, armed and supported in the field by Cuba and the Soviet Union.

It said Sunday that its intelligence services had monitored radio communications between rebel units inside Shaba and the Angolan capital of Luanda and that Spanish-speaking voices had been heard.

The Zairian government has called for assistance from France, Morocco, China, Belgium and the United States to help repulse the "invasion," the second in the past 14 months. Last March and April, France flew 1,500 Moroccan troops into Shaba Province to bolster the Zairian army after it failed to rout the rebels from a half dozen towns, including Mutshatsha.

The Congo National Liberation Front denied either that the Soviet Union or Cuba was helping its forces or that they had come in from neighborhing Angola. A sattement issued in Paris said the insurgents were living in the province and operating without outside assistance.

In Brussels, the Angolan ambassador there denied any involvement by his government in the Shaba fighting.

Supporting the Zairian thesis, in part informed sources here said yesterday that some of the rebels had been spotted earlier crossing a spur of Zambian territory that juts into south-western Zaire on their way to attack Kolwezi from Angola.

The Zairian government was said to be sending a special emissary to London to discuss the situation with Zambian President Kenneth Kaunda, who is visiting the British capital on his way to paying a state visit to Washington.

Last night, the Zambian government denied that Zambia had harbored any National Liberation Front forces on their way to Shaba Province from Angola.

Among reports reaching here from Shaba Province was some detail on the struggle for Kolwezi over the past weekend. At least five Italian-made Zairian jets were said to have been destroyed on the ground and airport facilities set ablaze.

The National Liberation Front said it had destroyed 13 Zairian planes and helicopters so far. The airfield at Kolwezi, which is almost 10 miles outsie the town, was said to be still in rebel hands as of late Sunday and Zairian Air Force jets failed to dislodge them despite two days of attacks. The Zairian jets are believed to be flying from Kamina, about 140 air miles.

In the attack on Kolwezi, which has a population of about 35,000, the rebels reportedly came in two columns from the east and southwest and initially overran the army headquarters, the mining company's hospital and other strategic points. It also blew up the army's ammunition depot. Later, Zairian troops were said to have retaken the headquarters and hospital.

Most of the town and the airfield were apparently still in rebel hands Monday and their use of urban guerrilla tactics were reported to be complicating the army's efforts to recapture lost ground.

All indications here were that the rebels were making considerable gains in consolidating their positions while the Zairian army was having trouble regrouping to launch a successful counteroffensive. One report said that a relief column with armored cars was halted after hitting mines on its way to Kolwezi. Some of the cars may have even fallen into rebel hands.

Rebel tactics have shifted from last year and they now appear to be going immediately for the major towns in an apparent attempt to deliver a severe psychological blow to the Mobutu government and attract popular support.