Guerrilla leaders Robert Mugabe and Joshua Nkomo today moved mush closer to agreement with Britain to end their war against the present government of Zimbabwe-Rhodesia and to participate in new elections there.

In a rare joint press conference here, Mugabe and Nkomo significantly softened their demands for power sharing and integration of their troops with the opposing forces of the Salisbury government during a British-supervised transition to legal independence for Zimbabwe-Rhodesia.

They also signaled their willingness to drop their demand for a U.N. peacekeeping force to monitor a ceasefire during a transition period. The Patriotic Front leaders indicated they now would accept another British-proposed international group.

These modifications considerably closed the gap between the guerrillas' demands and new proposals made in the British transition plan yesterday for mechanisms to protect the guerrillas' interests during the cease-fire and an election campaign to establish internationally recognized black majority rule there.

[Zimbabwe-Rhodesian air and ground forces meanwhile killed 60 Patriotic Front guerrillas and three Zambian civilians during a raid into Zambia Friday and Saturday, military haedquarters in Salisbury announced. The attack was the third since the start of settlement talks in London.]

The generally conciliatory tone adopted by Mugabe and Nkomo here today raised hopes for a binding peace agreement to the highest point yet in the two-month-old British-run Rhodesia settlement conference.

Mugabe and Nkomo were still critical of some of the British proposals. They said they want to negotiate further on key points when the conference resumes on Monday and declined today to give a yes or no answer to yesterday's request by the British for acceptance of the transition plan.

Lord Carrington, Britain's foreign secretary and the conference chairman, may yet have to deliver an ultimatum by the middle of next week to force at least conditional Patriotic Front acceptance of the plan, which has already been agreed to in principle by the biracial government to Zimbabwe-Rhodesian Prime Minister Abel Muzorewa.

Barring a drastic change in Patriotic Front tactics, however, the conference is now likely to move late next week to its next and last phase -- negotiating specific arrangements for the cease-fire.

That would leave less than a week to reach a final agreement before Nov. 15, when both Prime Minister Margaret Thatcher and President Carter must decide whether to continue their countries' economic sanctions against Zimbabwe-Rhodesia.

If agreement has not been reached but negotiations are still progressing by Nov. 15, Thatcher is prepared to extend temporarily the sanctions imposed after Rhodesia's former white minority government illegally declared its independence from Britain in 1965. The sanctions have been continued against Muzorewa's five-month-old biracial government because of the control the white minority kept over the civil service, courts, police and military.

Today Mugabe and Nkomo made no mention of their previous demand to share power with Muzorewa during the transition. The British plan provides for a British governor to take power from Muzorewa's government and run the country and conduct the election with several hundred temporarily assigned British officials and the existing Salisbury civil service.

The Patriotic Front leaders did not argue with British proposals for an all-parties election commission and a large team of observers from the British Commonwealth to monitor the election.

They also did not press their previous objections to the British governor's use of the present Zimbabwe-Rhodesian police to maintain order during the election. The British have told the Patriotic Front that British police officials could help the governor supervise the Salisbury police during the transition and British Special Branch bodyguards could protect Patriotic Front political leaders when they returned to Zimbabwe-Rhodesia from exile for the election campaign.

Mugabe and Nkomo continued to press, however, to extend the two-month transition period after an agreement here in which the election would be held. Reiterating their demand for six months they said enough time was needed for the guerrillas, their relatives and other refugees to return from neighboring black African "front-line nations," resettle and vote.

The British have promised to help transport the refugees back into Zimbabwe-Rhodesia as quickly as possible and have agreed to start the two-month transition period some time later on the date of the actual cease-fire, which would be negotiated in the next phase of the conference here. c

Mugabe and Nkomo said they remained most concerned about security during the transition. They were also skeptical about British assurances that the guerrillas would not be endangered or their voters intimidated by the Patriotic Front armies, which are to have equal status with the Salisbury forces under the British governor. However, Mugabe and Nkomo moved away from their previous demands that the opposing forces be fully integrated.

"We may not be able to integrate the total forces," Nkomo said today, adding that he now was seeking "a nucleus of equal numbers from both sides, even a small unit, to monitor the cease-fire" and "help ease the minds of the young people who have been fighting the war. We want them to see men who instead of shooting each other, are driving around with each other, monitoring the cease-fire."