The United States and major allies agreed today to seek resolution of the Namibian conflict by negotiating a constitution and other gurantees intended to persuade South Africa to stop blocking independence for that largely black territory.

The plan, reflecting the views of the Reagan administration, was approved by Secretary of State Alexander M. Haig Jr. and the foreign ministers of the four other countries that have been acting as the "contact group" seeking to prevent Namibia from becoming a crisis issued between South Africa and black African nations. The other nations involved in the diplomatic effort are Britain, France, West Germany and Canada.

At issue is how to achieve independence for Namibia, a thinly populated but mineral-rich land between South Africa and Angola, in a way that will satisfy black African aspirations while easing South Africa's fears that the new country will come under the domination of forces that it charges are hostile and communist-dominated.

South Africa has controlled the territory since the end of World War I. The U.N. Security Council in 1978 adopted Resolution 435 calling for independence under a U.N. timetable, but South Africa has been blocking progress, saying that the United Nations is too sympathetic to the Southwest Africa People's Organization, which has been waging guerrilla war against South African Forces in Namibia, also known as South West Africa.

The Reagan administration, which wants to repair the badly frayed U.S. relations with the white-controlled South African government without antagonizing the black African "front-line states" on Namibia's other borders, took the lead in working out the new plan. It evolved from extensive consultations carried out in Africa and Europe by Chester Crocker, President Reagan's nominee to be assistant secretary of state for African affairs.

The administration hopes the plan will permit a new start on the negotiations, lead to an independence agreement that will ingratiate Washington and its allies with the South Africans and the front-line countries and ease tensions to permit the withdrawal of Cuban and Soviet forces from Angola.

Tonight, ministers of the contact group met on the eve of the North Atlantic Treaty Organization meeting here and approved a statement reaffirming that Resolution 435 should be the basis for pursuing Namibian independence.But it also called for "ways to strengthen the existing plan" by adding to it "measures, including consitutional arrangements, with the aim of enhancing prospects."

U.S. officials have said privately that the idea is to work out in advance a constitution and safequards that will overcome South African fears of a U.N. bias toward SWAPO and convince South Africa that other forces in Namibia will have a fair chance at gaining power when the territory elects its own government.

So far, U.S. and other Western officials have been vague about how this might be done in ways that will gain the cooperation of the black African states and allay their concern that the plan is not a ruse to install a South African puppet government.

On another matter, State Department spokesman Dean Fischer characterized as "premature" reports today in The Washington Post and other U.S. newspapers that Haig will promise NATO that the United States will begin negotiations with the Soviet Union this year on reducing intermediate-range nuclear missiles based in Europe.

The published reports said a "preliminary commitment" to offer such a promise was decided on by the National Security Council on Thursday.