Britain and the United States are formulating plans for sending an international peace-keeping force, composed primarily of Commonwealth troops, to prevent a civil war in Rhodesia during a transition to black-majority rule, the British Foreign Office said yesterday.

A spokesman said the plan, termed a "critical element" in a Rhodesian peace package, was discussed by Foreign Secretary David Owen at the recent Commonwealth summit conference in London and with Secretary of State Cyrus Vance in paris last week.

The London Daily Mail said Canada, India, Ghana and Nigeria have agreed to take part, but the spokesman said he could not confirm the report.

In Washington, State Department spokesman said U.S. troops would not take part in the force. "We are not going to set up a peace-keeping force," he said. "This is not a proper activity for this government." He did not elaborate.

The British spokesman said the force would, be used only if both Prime Minister Ian Smith's government and the black nationalists agreed to a negotiated settlement.

If the plan is completed in time, Owen hopes to take it with him on a southern Africa tour he is planning for the end of July, he said.

John Graham, a senoir Foreign Office Official is to arrive in Lusaka July 3, accompanied by Stephen Low, U.S. ambassador to Zambia.

In Salisbury, the Rhodesia government issued a terse statement in reaction to the Foreign Office's comments:

"A government spokesman said the Anglo-American consulative group is expected in Salisbury in the latter part of next week, when discussions will resume on all aspects of a settlement issue."

The Rhodesia military command said yesterday that goverrnment troops had killed 42 black insurgents in Clashes Sunday and Monday.

The government also reported that 1,754 whites emigrated from Rhodesia in May, and only 415 entered, for a net loss of 1,339 - more than in any other month since before the white minority broke away from British rule in 1965. In the first five months of this year nearly 5,000 whites left the country.

In other areas of southern Africa yesterday:

Zambian President Kenneth Kaunda said his security forces had shot down a South African plane that violasted Zambian air space. South Africa denied the claim.

Twenty-six young blacks were fined amounts varying from $24 to $48 each for having taken part in a march on police headquarters in Johannesburg Thursday.