South Africa warned tonight that it might send troops into Zimbabwe in pursuit of suspected insurgents after four land mine explosions in two days wrecked vehicles in a military area near the Zimbabwean border.

The land mine explosions injured a civilian truck driver and his passenger yesterday and four soldiers today.

Pretoria said the mines were laid by African National Congress insurgents who crossed the border from Zimbabwe, and it has formally warned Prime Minister Robert Mugabe's government not to allow attacks to be launched from its territory.

Meanwhile, as widespread racial violence continued in many parts of South Africa, residents of the black township of Mamelodi, where 13 persons were killed when police opened fire on a crowd of demonstrators last week, told reporters today they had been tear-gassed and shot at from a helicopter.

The residents made this and other allegations at a news conference called to give their version of the shooting to correspondents, who were barred by new press laws from entering Mamelodi to report the incident. Police have admitted that a spotter helicopter was used, but denied that shots were fired from it.

The land mines have all been detonated by vehicles traveling along dirt roads on land belonging to the South African Defense Force west of the border town of Messina.

According to a Defense Force announcement, the civilian driver of a heavy truck and his passenger were severely injured, and a farmer escaped unharmed, when mines exploded under their vehicles yesterday. Two more mines, reportedly made in Czechoslovakia, were found later yesterday when troops swept the area.

Today, a third explosion in the same area wrecked a troop carrier, injuring four soldiers. Soon afterward a police vehicle detonated another mine, but no one was injured, an official statement said. Defense force authorities said they had found tracks that showed that the mines had been laid by insurgents who crossed the Limpopo River border with Zimbabwe. If true, this is the first time South Africa's northern neighbor has been used as an infiltration route by the guerrillas. South Africa has charged in the past that insurgents infiltrated its territory from four other black-ruled neighboring countries -- Mozambique, Swaziland, Lesotho and Botswana -- and it has launched reprisal raids against all except Swaziland. Mugabe's declared policy is that while his government deplores South Africa's segregationist policy called apartheid, and will have no formal relations with Pretoria, he will not permit the African National Congress to use his country as a launching pad for attacks. After yesterday's land mine blasts, Foreign Minister Roelof F. (Pik) Botha announced that he had cautioned the Mugabe government to ensure that no attacks were launched from Zimbabwe. After today's explosions, he issued a blunt warning that if Zimbabwe did not take urgent steps to prevent further land-mine explosions, "South African security forces will have no other choice but to follow the tracks themselves."

At today's press conference on the Mamelodi shooting, Smangaliso Mkhatshwa, a Catholic priest and patron of the activist United Democratic Front, said 44 persons had been killed and 1,200 arrested in police action in this and other black townships near Pretoria since July. This far exceeds official casualty figures.

Mkhatshwa and other speakers at the press conference repeated earlier accounts of the Mamelodi shooting last Thursday, saying a crowd of about 50,000 had stampeded when police opened fire on them, first with tear gas, then bullets. They said there were many elderly women in the crowd and some had been trampled to death. Others were crushed against a fence and choked on the tear gas, the residents said.

After the shooting, the residents said, police and troops had gone from house to house arresting people they said were agitators. They said there were many instances of brutality during these searches.