Britain anounced today that it is assembling a small international military force to monitor a cease-fire in Zimbabwe-Rhodesia between Patriotic Front guerrillas and the Salisbury government of Prime Minister Abel Muzorewa.

The force, which is expected to consist of more than 1,000 men from Britain and both black and white countries of the British Commonwealth, represents a major concession by the British in search of an agreement in the 9-week-old peace conference here.

The Patriotic Front, which has sought an international peacekeeping force to protect its interest during the cease-fire, welcomed the British move.

At the same time, guerrilla leaders Robert Mugabe and Joshua Nkomo formally declared that the British proposals for the transition to legal independence with black-majority rule in Zimbabwe-Rhodesia still "do not satisfy our essential requirements."

Despite sharp criticism of the British proposals, however, the language of the Patriotic Front indicated that Mugabe and Nkomo were still willing to make concessions.

In particular, they did not specifically demand a six-month transition period, although they reiterated that the proposed two months was too short for them to move all their exiled supporters from neighboring African countries back to their homes in time to campaign and vote in new election.

The Front document repeated charges that the British plan favored Muzorewa's government during the projected election campaign. It apparently was prepared over the past week, before today's reevaluation on the military force to monitor the cease-fire and before the current visit here by Zambiam President Kenneth Kaunda.

Kaunda, who has been meeting here with the Patriotic Front leaders and British officials, discussed the ceasefire at length today with Prime Minister Margaret Thatcher.

Although British were at pains to say they had always encouraged some form of monitoring the cease-fire, it was the first time Britain has proposed the introduction of any outside military force in the war-torn country.

British officials were careful to say the troops would not be "a fighting force" and that "intervention" in possible flare-ups between the two sides in the 7-year-old Rhodesian war was not envisaged.

The distinction is that although the troops could carry sidearms, they would not comprise a heavily armed peacekeeping force. The distinction is important in Britain, since several hundred British soldiers have been killed trying to keep the peace during sectarian strife in Northern Ireland.

It is also a senstitive problem for Muzorewa. He has accepted the British transition plan, calling only for the present Salisbury police, with British police supervision, to maintain law and order during the interim period. The role of the white-led Salisbury police was opposed by the Patriotic Front.

British officials provided few details about the monitor force, saying they would await discussion of specific cease-fire arrangements in the next and last stage in the conference to settle the 14-year-old Rhodesian problem. They did not say Britain, Australia, New Zealand and Fiji would be among countries providing troops if a cease-fire is arranged.

The British annoucement was made only after Australian Deputy Prime Minister Doug Anthony said in Canberra that Britain had asked his country to provide 10 teams of 11 men each to form part of the monitoring force.

Commonwealth officials suggested that among other possible contributors of troops from the group of former British colonies are India, Malaysia, Kenya, Sierra Leone, and Sri Lanka. British sources indicated that Britain would provide about 500 troops, with slightly more coming from the other countries.

Commonwealth Secretary General Sonny Ramphal said at least 1,000 troops would be needed. Ramphal, who has had sharp exchange with Carrington over the conduct of the peace conference, said he was happy that the British "are coming around to our point of view," even though they had not consulted on the monitoring force.

Zambian officials said Kaunda regarded the move as a "step forward." One official said, however, that far more than 1,000 troops would be necessary. "Four, five or six thousand or maybe more troops" would be needed, he said.

Kaunda's talks with Thatcher were described as "constructive and useful." Kanuda, whose country has come under increasing military and economic attack by Zimbabwe-Rhodesia for its support of the guerrillas, plans further talks Saturday with British and front leaders.

Mugabe, coleader of the Patriotic Front, noted that the troops issue had not yet been discussed in the peace conference itself, but added, "if it is we will consider it." The Patriotic Front had originally insisted on a United Nations peacekeeping force but began backing away from that last weekend.

In addition to the military men from Commonwealth countries to monitor the cease-fire, plans call for an unspecified number of Commonwealth civilians to observe the election, several hundred British officials to supervise and augment the Salisbury government's civil service and police force under a British governor, and British police bodyguards to protect candidates.

A ranking diplomat already has been chosen to be the British governor, although his identity has not been revealed. Other British civil servants also are being selected for duty in Salisbury. Enabling legislation for the transition period is being rushed through Parliament here.

Mugabe and Nkomo complained today that the British are doing all this to hurry the peace conference to completion without negotiating over the front's remaining objections to the British transition plan. British officials said the advance preparations do not prejudge the conference's outcome, but are necessary to set up the interim British government in Salisbury immediately if an agreement is reached.

Carrington is still seeking Patriotic Front agreement to move from the transition plan onto detailed discussions of cease-fire arrangements. But the guerrilla leaders are stalling, both to win further concessions from the British on the transition proposals and apparently to allow more time for their troops and followers to position themselves to enter Zimbabwe-Rhodesia from their bases in neighboring countries.