Nationalist leader Robert Mugabe canceled an appearance at a major political rally in southwestern Rhodesia at the last minute today, amid growing fears of a deterioration in the country's security situation in the 10 days before the national elections.

Mugabe's own security people told him they could not guarantee his safety from the airport into the city of Bulawayo. Just yesterday police discovered a weapons cache near that city. Mugabe has been the target of two assassination attemps in the last two weeks, including a 90-pound bomb explosion on his route to an airport after a rally Feb. 10.

Concern about security grew following the disclosure that two members of the government-controlled Rhodesian security forces were killed in a car bombing Thursday. Police had theorized that the deaths were linked to explosions at two Salisbury churches the same night.

The announcement by the military last night led to widespread speculation that the security forces carried out the church bombings to discredit with African voters the Marxist-oriented parties of Mugabe and his fellow guerrilla leader, Joshua Nkomo.

The white establishment and military have long claimed that Nkomo and Mugabe would suppress the churches, charges the two leaders have denied.

Balcks have long accused the security forces of terrorist acts, but the 200,000 white-majority comunity insists that "the Rhodesian security forces have never engaged in terrorist actions," as a police spokesman said after the wave of bombings.

The possibility that the security forces were involved could cause major problems for Britian's temporary colonial governor, Lord Soames, and lead to further violence in the closing days of the election campaign. One observer said it could provide the opportunity for groups to carry out bombings and then blame them on the military.

The cryptic anouncement by the military gave few details. It said that the two soldiers were killed in the car-bomb explosion about an hour after they reported to headquarters that they were tracking two suspected guerrillas in the Salisbury black township of Harare.

Police had speculated Friday that the two men were found in the car, then unidentified, had planted the bombs at the two churches and then had been blown up by explosives they were carrying to another site. The car exploded outside a church. Friday's announcement made no mention of the soldiers.

An unexploded bomb was discovered Friday at a fourth site, another church, and it bore slogans favoring Mugabe and symbols of his party. At the time, a spokesman for Nkomo's Patrioric Front party, noted that the slogan, written in the Shona language, was ungrammatical and cited this as evidence that the bombings were the work of the Selous Scounts, and elite unit of the Rhodesian Army.

In announcing the cancellation of the rally attended by about 35,000 people today, a Mugabe party official said unnamed forces were trying "to cripple our movement."

Although the military refused comment, it is known that one of the two dead soldiers, Lt. Edward Ngwarayi Piringodo, 27, was a Selous Scout. He was one of the few black soliders to have received the Silver Cross, the second-highest honor in the Rhodesian military. He was said to have been a reconnaissance expert.

The other soldier, Cpl. Morgan Moyo, also 27, reportedly was a munitions specialist.

The military and police refused to answer numerous queries by the press.

Although the British are supposed to be in control of the government, there have been frequent accusations that they must accede to the demands of the Rhodesian military, led by Gen. Peter Walls, because of the precarious security situation.

The governor has deployed only the Rhodesian security forces to assist the police in maintaining law and order, although the 22,000 Mugabe and Nkomo guerrillas gathered in assembly camps under the terms of December cease-fire agreement are also legally the forces of the governor.

Critics say the British have tilted too far toward the whites and the politician most of them support, former prime minister Bishop Abel Muzorewa. p